There is a story about two men repairing a fuel system. The young one, new to the job, remarked that the fuel smelled awful. The older one smiled and answered, “It may smell awful to you but it smells like money to me”. Fuel is an expensive part of aviation operations. In the life of an aircraft, fuel is the single greatest expense.

You would never throw away money, would you? Well maybe you do and possibly more than you realize. There are many ways to be “penny wise and pound (or dollar) foolish” in our industry. Our industry’s habits are rooted in the past. Many people still treat fuel as if it cost 10 cents a gallon. Do you?

WASTE FUEL = MONEY

When you decide to call fuel “WASTE” or “SLOP”, use your head and not tradition. A lot of people now find themselves paying big money to have this fuel removed and disposed of. Remember, the moment you call it “waste” or “slop” fuel, it may legally become HAZARDOUS WASTE! What’s the difference? Plenty! You will likely not be ALLOWED (by law) to recover this fuel — even for diesel or fuel oil use. You should call this fuel either “fuel” or, if you must, “downgrade fuel”.

DEFINITIONS

Waste (slop) Fuel: This is fuel unacceptable for aircraft or ground equipment use and must be disposed of accordingly. Waste fuel cannot be reclaimed or recovered by the facility operator for aircraft use. Waste fuel is generally contaminated fuel resulting from exposure to biological degradation, surfactants, oily water separator systems, chemicals, product mixtures, yard (surface) drains, spills, or from other various contamination sources that can be detrimental to fuel quality and flight safety.

Sump and Sample Test Fuel: This is reusable fuel removed from facilities and equipment while performing routine quality control testing or when performing fueling equipment maintenance. Sump (sample/test) fuel is generally considered aircraft quality after removing small quantities or traces of water and/or solid particles that may result from testing or draining of equipment. Fuel removed from filter vessel drains, jar and bucket sampling, filter membrane testing, sump (tank bottom) drains on fuel storage point vents, low point drains, and other fuel samples from locations where the fuel is generally considered clean and dry, is usually of excellent quality. This fuel can be returned to the storage tanks (you may want to filter it first) or use as a ground equipment fuel.

IMPORTANT: Sump/sample fuel becomes waste fuel only if it becomes unacceptable for aircraft or ground equipment use. The point here is that there is, most likely, nothing wrong with at least 99% of that fuel. Think about it! In most fuel systems, your samples are all perfectly clean and pure. IS IT REALLY NO GOOD WHEN IT COMES OUT OF THE FILTER DRAIN PERFECTLY CLEAN?

Many people consider the fuel from a filter membrane test (Millipore® or MiniMonitor®) to be slop fuel. The truth is that it is surely the cleanest fuel you have. The membrane is a VERY fine filter. Fuel released from air eliminators and pressure relief valves is clean fuel. Sump samples from tank or filter drains rarely contain more that traces of dirt or water.

You not only lose the cost of the fuel, but the waste hauler’s charge and your mark-up, because you could have sold that fuel. We have heard of people having to pay over $3.00 per gallon to have waste fuel removed.

A common practice is to run pipes from filter accessories (air eliminators, pressure relief and automatic water drain valves) to the slop tank. Eventually these items leak, filling the slop tank or oily water recovery system quickly. We know of a location where over 1,000,000 gallons/year was wasted in this way. The use of inline flow indicators can help you discover such leaks.

SIMPLE SOLUTIONS

We recommend you spend a little time and money now, to save a lot of money later. BE SURE YOU CONTACT YOUR AIR LINE AND OIL COMPANY FUEL QUALITY PEOPLE FIRST! Be sure to meet all state and local requirements with any system changes.

1. Connect the air eliminator and pressure relief valve outlets together with pipe or metal tubing and run this back to the storage tanks (DO NOT DO THIS WITH AUTOMATIC WATER DRAINS). If you have above-ground tanks, you must mount a check valve at the tank.
2. When you drain a filter to change elements, take sump samples (start with a clean bucket) until you get a clean sample, then use a CLEAN hose, tubing , bucket or other means to put the rest of the fuel back into the storage tank. If you have underground tanks, you can install two manual drains. Connect the extra drain valve to the air eliminator and pressure relief tube we recommended in Solution #1. It might be a good idea to use a self-closing, spring-loaded valve so that it will be unlikely to be left open. Then you can drain the vessel very easily and cleanly, directly to storage.
3. Filter membrane samples (MiniMonitor® or Millipore®) are collected in buckets. If you clean the bucket before the test, you can return the fuel to storage. If you use automatic drain systems, they should be connected to the slop tank or a sump separator. If you run the auto-drain to a slop tank, it is a good idea to mount a flow indicator on the pipe so you can see if the valve leaks.
4. Sump samples (white bucket) can have dirt or water in them, but usually no more than traces. Start with a clean bucket and after the test you can let the bucket settle a half hour and return a majority of the fuel to storage, then pour the sediment into the slop tank or drum.
5. Review your operations carefully, and check your slop removal costs. As you can see, a majority of these changes cost very little to implement. Many military services, air lines, and oil companies follow such practices. We recommend you re-think your policies if you are throwing money away. Also, let’s face it: even with our knowledge of environmental damage, some people still dump fuel on the ground. WE MUST CHANGE THIS HABIT!

Bucket samples or fuel drained from a filter separator must never be allowed to splash into a storage tank because of the danger of an electrostatic explosion. However, this operation can be performed safely by using a connection added to the storage tank fill piping (if you have an underground tank). If you have above ground tankage, this problem can be handled by a Sump Separator with a pump).

We would be remiss if we didn’t mention our Sump Separator fuel recovery unit (see our Bulletin 22).

The Sump Separator fuel recovery unit is basically a heavy gauge steel tank, bright white epoxy lined, with a cone bottom. It is specially designed to serve as fuel recovery unit, reducing “slop” to a minimum. The Sump Separator valve mechanism is specially designed to concentrate sediment and water. You can run air eliminators, pressure relief, automatic water drains, etc. to a sump separator. You can also pour white bucket samples and filter membrane test buckets into your sump separator. After settling, you can recover at least 95% of the good fuel. Options include filters, pumps, etc., so you can be confident of not putting dirt or water back into the storage tank.

Following these practices can reduce your wasted fuel and save money.

NOTE: If you don’t want to reuse this fuel as jet fuel, you may consider using it as fuel for diesels or oil furnaces. At many airports, the only fuel used in diesel refueler trucks and tugs is Jet A. Consult the manufacturer of the furnace or engine. You may want to blend jet fuel with diesel for engine use. Also, if you use it as motor fuel, for use in vehicles which go on the public roads, you will have to keep careful records and pay taxes. You will still save a lot of money.

MINIMONITOR IS A REGISTERED TRADEMARK OF GAMMON TECHNICAL PRODUCTS, INC.
MILLIPORE IS A REGISTERED TRADEMARK OF THE MILLIPORE CORPORATION

The fuel pressure that reaches the aircraft is simply a matter of how much a valve has been opened. Right?

But how much a valve is open controls flow rate! Right?

Then how can the same control valve regulate downstream pressure and also the rate of flow? This question puzzles many people, so we thought this would be a good subject for a GamGram.

First of all, let’s review how a control valve works. Diagram A shows the conventional diaphragm valve made by many manufacturers, such as Cla-Val, Watts (formerly named Muesco), OCV, etc.. Diagram B shows the same valve in piston configuration which is made by Brooks/Brodie and Thiem/Whittaker. Diaphragm and piston valves both work the same way so we will select only one, the diaphragm type, for this discussion.

The only action a control valve can take is to open or close. How much the valve is open is totally dependent on how much fuel is trapped above the diaphragm or piston (see the shaded area). Therefore, if the cavity above the diaphragm has the maximum amount of fuel in it, caused by upstream pressure being directed there, the diaphragm is forced downward and the valve G is closed (or valve D in the piston version). However, if you permit a small amount of that fuel above the diaphragm or the piston to be bled off to the downstream side, the valve will open slightly. If you allow the cavity to be completely vented to the downstream side, the valve opens wide.

This is done automatically using small auxiliary valves called “pilots”. Different pilot valves perform different functions; rate of flow control, slug, pressure control, etc.. As pilot flow increases, the lower the pressure on the main valve diaphragm and the main valve opens. As pilot flow decreases, the opposite happens.

A Rate of Flow Control pilot B senses the flow rate by measuring the pressure difference across the orifice. Tubing connections are located upstream and downstream of the orifice plate. These 2 pressures, P1 and P2, are compared across a diaphragm that has a stem connected to a valve. If this pilot diaphragm moves because of a change in pressure drop, the valve connected to the stem opens or closes to regulate the amount of pilot flow, and therefore, the amount of fuel above the diaphragm H in the main valve. This regulates the flow rate through the main valve. Actually, the rate of flow pilot is named incorrectly by all manufacturers; it should be called a “flow limiting pilot” because it simply prevents flow from exceeding a desired limit. This can be adjusted by turning a screw F that regulates the spring force on the pilot diaphragm. Think of it this way; the spring force makes up the difference between the 2 pressures (P1 upstream and P2 downstream) of the orifice plate.

The Pressure Control pilot C is also usually a diaphragm valve that senses the pressure downstream of the main valve and compares it to the force of a spring that is on the other side of its diaphragm. Just as in the rate of flow pilot, there is a stem connected to the diaphragm that adjusts a valve that is connected to it. This valve regulates the amount of fuel above the diaphragm in the main valve by regulating the flow rate through the pilot system. Therefore, if more pressure is required, the pilot valve opens slightly to let more fuel leave the space above its diaphragm. This allows the main valve to open a bit to build the downstream pressure to the desired level. If downstream pressure gets too high, the pressure pilot closes slightly, preventing escape of fuel from above the main valve diaphragm, forcing the main valve to close slightly, and therefore, reducing downstream pressure.

The question we posed at the start was, “How can the same valve control pressure and rate of flow?” The answer is that the manufacturer arranges the 2 pilots in series with the rate of flow pilot first, as shown in the diagrams. Therefore, if the flow rate is below the desired level, the rate of flow pilot will open to allow more pilot flow, thereby decreasing the volume above the main valve diaphragm to allow the main valve to open more.

The pilot flow proceeds to the pressure control pilot. If the downstream pressure is less than desired, it opens to allow more pilot flow, just as the rate of flow pilot did. However, the rate of flow pilot is in command because it already has opened to allow maximum main stream flow. Therefore, if the pressure control pilot opens to cause more downstream pressure when the flow rate is already at the maximum allowed by the rate of flow pilot, nothing happens. The rate of flow pilot is “the boss”. On the other hand, if the downstream pressure is greater than desired, the pressure pilot will close slightly to reduce pilot flow and cause more fuel to fill the space above the main valve diaphragm, causing the main valve to close slightly to reduce downstream pressure. But this also reduces main stream flow rate! So the rate of flow pilot opens more — and more – and more until it is wide open but the flow rate will not increase because the pressure control pilot is now “the boss”.

The remarkable thing about diaphragm or piston valves is that they can do many other tasks, simply by use of various pilot valves. For example, they make excellent check valves, fuel level controllers, back pressure regulators, slug and deadman valves. As an example, we regularly use one pressure control valve to control 2 different fueling pressures, simply by use of a manual selector valve. One position controls pressure for underwing fueling while the other position provides a much lower pressure for overwing fueling. In addition, we use a solenoid pilot to provide deadman control, only in the underwing mode. All this with one valve!

The most abused piece of refueling equipment is the hose. Do people enjoy “beating up” on hose? Or do they do it because it is the only major component that they can kick, bend and generally mistreat without it fighting back? The fact is that aircraft fueling hose is a remarkable product that can take a lot of abuse, but in some respects, it is delicate. In this GamGram, we are going to point out the astonishing things aircraft fueling hose can do, and we will also tell you how to treat it kindly.

First of all, consider speed. Would you believe that the conventional 300 gpm (U.S.) refueling rate for an airliner means a velocity of 19.2 feet per second (fps)? It’s true! That is nearly 3 times faster than the rate we design for steel pipe. And in deck hoses, many operators run at 500 gpm in 3” hose; that is 22.2 fps. The U.S. Air Force packs 600 gpm through 3” hose for a velocity of almost 27 fps. High velocities like this create turbulence that requires exceptional “adhesion”. (That is the property that determines how strong the bond is between the inner liner (called the tube), and the first layer of reinforcing cord). Poor adhesion results in the tube coming loose and being literally pushed right out of the hose body and into the fuel flow. Is it any wonder that all responsible operators insist on using fully qualified aircraft refueling grade hose, either to API-1529 (American Petroleum Institute) or to the British specifications BS-3158?

What about bending? Think of a piece of common rubber tubing having no reinforcing cord in it. Bend it sharply until it is double; it kinks. Let it straighten out, and it will look like new. Bend it again, and it will kink in a different place. Now think about bending a piece of fueling hose until it kinks. (Please use one that has been retired from service). It takes a lot of force, and notice that the cover is smooth and uniform on the outside of the bend, but it has fine ripples on the inside of the bend. Also, there are two points that stick out on opposite sides. (See Figure 1). When you release the force that caused the kink and let it straighten out, the hose stays out-of-round where it was kinked, sometimes for a long period.

It depends on how long the kink is held.

1. If a kink has been held very long, the hose will kink again and again at exactly the same place for the rest of its life.
2. Those two points that are on both sides of the kink are where the hose wall was bent absolutely double.
3. Those two points are where the hose cover will be worn through very quickly as it is dragged around on the fueling ramp.

What is the reason that a kink in a reinforced hose can become permanent? Rubber is flexible and it surely straightens out after it is bent. Rubber tubing does not take a permanent kink. The obvious difference is the reinforcing cord in the hose wall. We were puzzled over this phenomenon several years ago, and finally came up with an explanation that no hose expert has been able to knockdown. So here we go!

Consider the cross-section of hose in the photograph, (Figure 2). Now let’s consider what is really happening at the bend. The rubber on the outside of the bend (cover) stretches, but the thread will stretch only slightly with respect to the rubber, without taking a permanent set. The rubber that forms the tube of the hose bunches up because it cannot move anywhere. The thread is literally being stretched around the tube rubber. It is overstressed, overstretched and weakened.

Burst tests of kinked hose have not proven that the hose will fail at locations where kinks have occurred once or twice, but it is a well known fact that the hose will be worn through by being dragged on concrete where the kink points are located. However, if the hose has been kinked for a long period, a soft spot develops and bursting can occur.

Just how strong is aircraft refueling hose? The latest specification defines two grades, 1 and 2. Grade 1 has a working pressure of 150 psi while Grade 2 is rated at 300 psi. The burst ratings are 4 times higher – 600 psi and 1200 psi. These hoses must be pressure tested at 300 psi and 600 psi to conform to the newest industry requirements.

Think of it! The valves and pipe on a refueler truck are tested at only 225 psi which is 1½ times more than design pressure. But the hose is tested at up to 600 psi! Why is this so? In this author’s opinion, such a high pressure test is necessary to insure that the couplings are securely attached, but also, because this is the best way to find manufacturing flaws. Tests on used hoses find areas of weakness that have been caused by handling abuse and damage.

The main point we want to leave with you is that hose kinks simply must be prevented. Many times a nozzle storage receptacle will be found in a position such that the hose must be kinked for the nozzle to fit. Sometimes hose reels have fitting connections that force the hose to kink as it winds on the drum. We have seen severe kinks caused by personnel who pull out a hose at high speed. When the last coil of hose comes off the drum, the inertia of the fast turning drum makes it keep right on turning; actually pulling the shoe back. This abuse causes the worst kinks we have ever seen. In fact, we have seen small diameter hose broken in this way.

Hose must never be allowed to lie on the ramp with a complete loop. When the hose is pulled, a kink almost always occurs, especially if the end that is under the loop is pulled.

With reasonable care, hose can provide reliable service for many years. Abuse by personnel is the greatest single cause of shortened life.

One of the truths of quality control is that the better it is, the easier it is to take it for granted. When you have very few problems, it is hard to be as careful as you should, and the problems you do have are more difficult to anticipate. In any business, you do not announce your problems to the world, and unfortunately, this makes it difficult for others to learn from your mistakes. This GAMGRAM is a collection of stories about problems other people have experienced. You may learn from their mistakes. To avoid embarrassment, no names or locations are given. We have tried our best to tell these stories accurately, although some are based on second hand information. The problems are real and can happen anywhere.

It is not possible to find humor in these stories. The chance of disaster is remote. Usually several errors, failure or mistakes have to occur simultaneously for a disaster. The question is, could you live with yourself if you were responsible for death or suffering? Intentions are not important, performance is. One accident a year worldwide is one too many. We have come too close to major disasters in the past — it can happen.

BALANCED POLICY – In ancient times, it was policy to kill the messenger that brought bad news. We assume that not all bad news got through! CAN’T DO THIS! Review your methods, and don’t make the punishment for a mistake equal to the punishment for covering up a mistake. This especially applies to fuel spills and other legal and safety related situations. Don’t laugh, we’ve heard of people who tried to hide fuel spills to protect their jobs! In one example, an employee washed the fuel into a stream — resulting in a much larger mess to clean up. The government sees this as your fault and may levy larger fines!!

La forma más segura de evitar que un caballo no se fugue de su finca es manteniendo las puertas del establo bien cerradas. Piense en la válvula de control de presión de punta de la manguera en inglés Hose End Control Valve (HECV), esta sería la “puerta del establo”. Pudiera atar el caballo a una cuerda sujeto a una estaca, pero si la cuerda se rompe o se zafa, el caballo sale por la puerta si está abierta, Bajo esta analogía piense en la “cuerda” como el análogo de la válvula de control de presión en línea del vehículo de reabastecimiento de combustible.

En la actualidad las especificaciones para los vehículos de suministro de combustible se requieren dos controles para la presión de combustible separado e independiente: Uno respalda al otro en caso de fallo ¡La puerta y la cuerda ofrece mejores probabilidades! La válvula HECV es el método menos costoso de todos los dispositivos de control de presión en el mercado y se ha vuelto muy popular como control secundario (aunque en Europa la utilizan como control primario).

Esta válvula comenzó a usarse en la década de los años 60 al elevarse el consumo de combustible en aeronaves y el caudal de abastecimiento aumento en consecuencia, entonces la industria reconoció la insuficiente atención que se le estaba prestando al control de la presión durante el suministro y, en particular, a la prevención de los picos de presión en la aeronave. El primer producto en el mercado fue una boquilla de abastecimiento de gran dimensión y peso con un dispositivo de control de presión integral. Sin embargo el complejo dispositivo dio paso rápidamente a la HECV que conocemos en la actualidad. Existen al menos cuatro versiones disponibles de este dispositivo; pero en este boletín haremos hincapié en la variedad más popular que está hecha por Carter Ground Fueling, división de Argo-Tech y Whittaker, división de Meggitt.

Antes de describir el funcionamiento de la HECV, debemos aclarar un aspecto. Si quiere medir longitud, presión, peso, luz, etc., debe tener un punto de partida razonable, una “línea de base”, un dato específico; por ejemplo, la altura de una persona no se mide desde su nariz! entonces si deseas medir la presión en un sistema de combustible, el único punto de partida será la presión atmosférica. Esto sucede porque utilizamos ese dato como referencia o sea si un manómetro de presión de aceite en un motor indica 100 psi, eso significa 100 psi por encima de la presión atmosférica.

En los sistemas de combustible se debe usar la presión atmosférica como nuestra base o dato. Si la válvula HECV controla a presión de nuestro sistema a un máximo de 45 psi, esto significa que la presión del mismo está a 45 psi por encima de la presión atmosférica. Pero ¿cómo es que la válvula sabe cuál es la presión atmosférica? Lo explicamos, Obviamente esto se logra a través del puerto de venteo el cual canaliza la presión atmosférica hacia un lado del pistón sensorial.

Observe el dibujo y si del otro lado del pistón está actuando en contraposición la presión ejercida por el combustible, entonces estamos en presencia de un dispositivo para medir la presión y si le adicionamos un muelle en el lado de baja presión (presión atmosférica); entonces la presión del combustible es equivalente a la medida en que se comprime el resorte. Al tener la base establecida, entonces al observar el dibujo vemos que cuanto más se mueve hacia atrás el pistón sensorial también se comprime el resorte y resulta más pequeño el pasaje entre el labio F y el sello C por el cual el combustible debe pasar. El anillo exterior A conocido popularmente como el “pistón exterior” (aunque en realidad no es un pistón), está unido al pistón sensorial mediante una estructura de redes cruzadas (tipo cortador de galletas), al cerrar el pasaje lo que hace es cortar el flujo de combustible y con esto provoca una caída de presión: mientras más obstruye el flujo de combustible, menor será entonces la presión del combustible aguas abajo de la válvula.

Aclarado el funcionamiento y cómo acciona la válvula HECV, ¿qué pensaría de un operador que puso un tapón en el puerto de ventilación, porque detecto una fuga de combustible a través de ese puerto y quiso detenerla? ¡Realmente esto sucedió! y por supuesto, la válvula HECV luego de hacerle eso dejó de funcionar debido a que no tenía forma de acceder y detectar la presión atmosférica. Lo correcto que había que hacer en ese caso para detener esa fuga era reemplazar sello B el cual evita que el combustible pase a esa cámara.

Al retornar a la analogía del caballo en un corral y su comparación con la válvula HECV como la puerta que evita el escape. Donde dijimos además que la cuerda con que se ata el caballo podría considerarse en similitud a la válvula de control de presión en línea (ILCV). El funcionamiento correcto de estas dos válvulas se debe comprobar periódicamente en lo referente a la válvula HECV esto no resulta difícil porque todo lo que se necesita es un manómetro colocado en el puerto lateral de la boquilla o en una conexión aguas abajo.

Ahora analicemos cómo se pude verificar la válvula reguladora de presión en línea conectada en el sistema de suministro del camión. Este proceso es más difícil porque no hay puerto disponible para montar un manómetro antes de la HECV. Lo que se hace en este caso es simular una falla en la HECV como si no trabajara correctamente (se soluciona utilizando un bloqueador), y entonces podemos comprobar el trabajo de la válvula de control en la línea, siendo capaz de regular la presión en la boquilla de abastecimiento. Una forma sencilla de bloquear la acción de la HECV es a través de un tubo con desconexiones rápidas en ambos extremos del pistón que lo que hace es proporcionarle temporalmente la presión de combustible en el puerto de ventilación; de esta forma se evitá que el pistón detecte la presión atmosférica al tener la misma magnitud de presión en ambos lados del pistón sensorial por lo que no puede funcionar.

Desafortunadamente, este simple método para el ensayo crea una situación adversa pues hace que el puerto de ventilación (normalmente seco) se humedezca con combustible, por ello, una vez terminada esta prueba, es una buena práctica hacer funcionar la HECV varias veces para expulsar el combustible que pudiera quedar dentro de la misma por detrás del pistón; esto evita que en el futuro reciba reportes falsos de que la válvula tiene salidero a través de ese puerto. Para evitar este efecto secundario, en los últimos años, los fabricantes han ideado dispositivos de bloqueo que interfieren mecánicamente con el movimiento del pistón. No se necesitan tubos y tampoco se humedece el puerto de ventilación.

Ahora surge la pregunta de cómo determinar si el resorte esta con condiciones y no ha perdido parte de su fuerza o si alguno de los sellos en el HECV necesita reemplazo. Claramente, si el sello B tiene fugas, podemos ver fácilmente la fuga de combustible a través del goteo en el puerto de ventilación. Pero ¿cómo se prueban los sellos C y D?, Para esto se coloca un manómetro en el puerto lateral de la boquilla, o en la tubería aguas abajo con la ayuda de un banco de pruebas. La mejor manera de ejecutar estas pruebas es presurizar el sistema con una bomba manual de pistón, ya que le permite aumentar la presión de forma estable muy lentamente. Un equipo como el que se utiliza para comprobar mangueras (GTP-2157-A) sirve para este propósito, pero solo si se conecta aguas arriba del HECV y aguas abajo de la válvula de control en línea que está a bordo del vehículo. A medida que bombea el combustible, el manómetro del probador de mangueras y la presión de la boquilla serán casi iguales hasta que el pistón exterior toque el sello C. Si los sellos C, D y E no tienen fugas, a medida que aumenta la presión del probador de mangueras a 100 o 200 psi, la presión en la boquilla debe mantenerse con la magnitud correspondiente al tipo de resorte utilizado el cual es calibrado para diferentes presiones. Sí la presión de la boquilla se mantiene a una magnitud de más de 3 psi diferente al ajuste que esta calibrado el resorte, es posible que necesite un resorte nuevo.

Es ahora cuando corresponde el turno a otra de las partes de la HECV la válvula de alivio de presión señalada en el dibujo como E. Muchas personas que durante años han trabajado con la HECV no tienen idea de que esa pequeña válvula interna está ahí. Su función es permitir que la HECV se abra nuevamente después que esta se haya cerrado por completo al concluir el suministro. Puedes comprobar su funcionamiento en la prueba anterior liberando la presión de la manguera después de concluir el ensayo y observando el manómetro colocado en la boquilla de suministro. La válvula E, se abrira cuando la presión de la manguera ha caído a 30 psi aproximadamente, por debajo de la presión de ajuste del resorte.

Casi termina el boletín, y aún no hemos mencionado la mejor prestación que ofrece la HECV: nos referimos al control de picos de presión (golpe de ariete). Estamos en presencia de un dispositivo sencillo que no es mucho más grande que su puño. Tiene solo una parte móvil y no tiene controles externos, el cual está montado en la boquilla de abastecimiento de combustible donde detecta instantáneamente que la válvula interna de la aeronave se ha cerrado. Al ocurrir eso la HECV simplemente se cierra ¡muy rápido! evitando la propagación de la onda de presión. En el gran número de ensayos que he presenciado con la HECV, el aumento del pico de presión en la aeronave nunca ha superado los 90 psi. La mayoría de las especificaciones permiten 120 psi como máximo. Desafortunadamente, no puede medir un aumento del pico de presión con manómetros comunes, porque el mecanismo de los mismos no responde con la rapidez necesaria. Puede pensar que su manómetro muestra el aumento, pero no es así. La única forma en que se puede medir un aumento de presión es con un transductor o transmisor de presión que tenga una respuesta de alta frecuencia y mida la presión en corto espacio de tiempo. Este aumento se rastrea en un registro. Un fallo de la válvula en línea o de la válvula del hidrante puede ocurrir en sistemas modernos que tienen clasificaciones de 275 psi, pero con un HECV que pesa solo 2.3 lbs, puede proteger a la aeronave con efectividad. ¡Qué dispositivo tan fantástico!

Keeping the barnyard gate closed is the surest way to keep a horse from getting away. Think of a Hose End pressure Control Valve (HECV) as the “gate”. You could tether the horse to a stake with a rope, but if the rope fails, the horse leaves through the open gate. Think of the “rope” as the on-board pressure control valve of the refueling vehicle. The majority of equipment specifications today require that there be two separate fueling pressure controls; one backs up the other in case of a failure. The gate plus the rope gives better odds! The HECV valve is clearly the least expensive of all pressure control devices on the market and has become very popular as a second control.

The HECV came into use in the 1960’s when fueling rates were increasing and the industry recognized that it had really not paid enough attention to pressure control, and in particular, prevention of pressure surges at the aircraft. The first product on the market was a massive fueling nozzle having an integral pressure control device. This rather complex device gave way shortly to the relatively simple HECV that we know today. There are at least four versions available today; we will restrict this essay to the most popular variety which is made by Carter Ground Fueling, division of Argo-Tech and Whittaker, division of Meggitt.

Before we describe how they work, there is one thing we must get straight. If you want to measure something (length, pressure, weight, light, etc.), you must have a reasonable starting point, a “base-line”, a datum. You would not measure your height by starting with your nose! If you want to measure a pressure in a fuel system, the only logical starting point or datum is atmospheric pressure. If an oil pressure gauge on an engine reads 100 psi, that means 100 psi above atmospheric pressure.

In a fuel system, we must use atmospheric pressure as our “base” or datum. If a HECV valve is supposed to control at 45 psi, it must be 45 psi above atmospheric pressure. But, how does the valve know atmospheric pressure? Obviously, we must tell it — this is done in a HECV with a Vent Port that will duct atmospheric pressure to one side of a piston. See drawing. If fuel pressure is on the other side of the piston, we have “created” a pressure measuring device. By using a spring on the low pressure side, the fuel pressure is measured by how much the spring is compressed.

Now, with the matter of a base-line established, we see in the drawing that the more the Sensing Piston moves in compressing the spring, the smaller is the passageway for fuel to pass through between the lip F and the seal C. The outer ring A which is attached to the Sensing Piston with cross webs, is often called the “outer piston”, but it is clearly not a piston. As there is no proper word, think of it as a “cookie cutter” — it literally shears the fuel stream! It creates a pressure drop. The more it cuts off the fuel flow, the lower the downstream pressure.

Now that the action of the HECV valve is understood, what do you think of an operator who puts a plug in the vent port “because it leaked”? It has really happened! The HECV valve then became inoperative, because it had no way to sense atmospheric pressure. What should have been done was the replacement of the seal B.

Back in the first paragraph, we talked about the horse in the barnyard and how the HECV could be thought of as the gate. Then we said that a tether rope could be considered as the on-board pressure control valve. Periodically, we must check the performance of both valves. It is not very difficult to check the HECV valve, because all you need is a pressure gauge in the side port of the nozzle or on a fitting just downstream.

Think about how you can check the truck’s on-board control valve. This is more difficult because there is no convenient port to mount a pressure gauge. What we must do is simulate a condition of failure of the HECV, and then see if the nozzle pressure would be controlled by the on-board valve alone. A simple way to do this is with a tube having quick disconnects on both ends. It is called a Block-Out Device. All it does is provide you with a way to temporarily put fuel pressure at the Vent Port to keep the piston from sensing atmospheric pressure. With fuel pressure on both sides of the piston, the HECV is prevented from operating. Unfortunately, this simple test method creates an aggravation — it makes the normally dry vent port be wet with fuel. So after this test, it is a good idea to cycle the HECV several times to blow fuel out from behind the piston; this avoids false reports that the HECV is leaking.

In recent years, the manufacturers have devised block out devices that mechanically interfere with motion of the piston. No tubes are needed and there is no wetting of the vent port.

Which brings us to the question of how to determine if the spring has lost some of its force or if any of the seals in the HECV need replacement. Clearly, if seal B leaks, we can easily see fuel leaking from the vent port. But how do you test seals C and D? This requires a pressure gauge in the side port of the nozzle – or in the downstream piping of a test stand. The best way to run these tests is with a piston-type hand pump, because it allows you to increase pressure very slowly. A hose tester serves this purpose, but only if you connect it upstream of the HECV and downstream of the on-board control valve on the vehicle. As you pump, the hose tester pressure gauge and the nozzle pressure will be very nearly the same until the outer piston reaches seal C. If seals C, D, and E do no leak, the pressure in the nozzle should hold at the pressure rating of the spring as you increase the hose tester pressure to 100 or 200 psi. If the nozzle pressure holds at a level more than 3 psi different from the spring setting, you may need a new spring.

Here is where we learn about the pressure relief valve E. Many people who have worked with HECV’s for years have no idea it is there. What it does is allow the HECV to open again after it has closed. You can check its action in the above test by releasing hose pressure after the test and watching the nozzle pressure gauge. The relief will usually pop open when hose pressure has dropped to about 30 psi below the spring setting.

Now we are at the end of the page, and we still have not mentioned the very best feature of the HECV — pressure surge control. Here is a simple device, not much larger than your fist. It has one moving part and no external controls. You mount it on the fueling nozzle where it can sense instantly that the internal valve of the aircraft has closed. The HECV simply closes — fast! In test after test that I have witnessed on HECV’s, the surge at the aircraft has never gone above 90 psi. Most specifications permit 120 psi as a maximum. Unfortunately, you cannot measure a surge pressure on an airport with common pressure gauges, because the mechanism will not respond quickly enough. You may think your gauge shows the surge, but it is telling you a lie. The only way a surge pressure can be measured is with a pressure transducer having high frequency response. The surge is then traced on a recorder.

Failure of the on-board valve or of the hydrant valve may occur in modern systems having ratings of 275 psi, but with a HECV weighing only 2.3 lbs, you can positively protect the aircraft. What a fantastic device!