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!

On nearly every refueler truck and hydrant cart which is capable of underwing (pressure) refueling there is a gauge marked “nozzle pressure”. Since it is not connected to the nozzle, how can it really show actual nozzle pressure? There is no magic involved. Actually, it does not show nozzle pressure! What it shows is simulated nozzle pressure. The gauge is connected to a device called a VENTURI. It is usually mounted at the inlet of the hose reel. Few people understand what the venturi does or how to adjust it properly. A large number of the venturis in the world are not properly adjusted.

To understand a venturi, we have to understand energy. In a flowing pipe, the energy at any point depends on velocity and pressure. The energy level remains the same except for gradual decreases caused by friction as the fluid passes through valves, fittings and pipe. In Figure 1, the pressures at A, B and C will be exactly the same if there is no flow. If flow takes place, the pressure at C will be nearly the same as at A, less a friction loss. However, the pressure at B will be much lower than at C. The reason is that we turned some of the pressure energy into velocity energy to get the fluid through the small opening. After the restriction, the velocity energy decreases so the pressure energy must increase accordingly.

Giovanni Venturi, the Italian scientist, discovered 200 years ago that he could make an excellent flow meter with very little pressure loss if he carefully designed the outlet after the restriction to avoid turbulence and friction. See Figure 2. The pressure recovery zone is a long gradual taper back to the original pipe size. The pressure drop (energy loss due to friction) in his design is much less than in Figure 1.

The purpose of a venturi in a fuel system is not to measure flow rate but to provide a simulation of nozzle pressure at B and to use that pressure as the control signal for the pressure control valve. When fuel is pumped to an aircraft, you want maximum flow, but pressure must be kept below 50 psi maximum aircraft fueling pressure to achieve maximum flow rate into the aircraft. See Figure 3. Such a system may require 80 psi at point C in order to result in 50 psi at the aircraft (D), assuming a 30 psi pressure drop in the reel and hose. The problem is that when the aircraft is full, there is no flow and therefore no pressure drop. Without a venturi such a system would, at no flow, put 80 psi on the aircraft. This could cause damage to the aircraft fuel system. If set at 50 psi, the actual fueling rate would be very slow, no where near the design rate because the pressure drop between points C and D would use up much of that 50 psi.

FIGURE 3

(The above example shows a simple, non-compensated single pressure control valve system, for simplicity.)

The venturi system (Figure 4) solves this problem. By locating the venturi just before the hose reel, pressure drop across the filter and meter is ignored by the pressure controller. The venturi’s job is to compensate for pressure drop in the hose reel, hose and nozzle. As flow increases, the venturi “fools” the pressure control valve into delivering enough pressure so that there is 50 psi at the low pressure point (B). The venturi is “tuned” so that B and D match. Under full flow conditions, supply pressure (C) may be 90 psi and pressure at point A may be 70 psi, but pressure at the nozzle and at point B is 50 psi. When the aircraft is full and flow stops, pressure A, B, C and D return to 50 psi.

FIGURE 4

Venturis are not designed to match each system because it would be far too expensive to custom machine each one. Instead, a venturi is selected that has too much compensation. The Whittaker (formerly Thiem) F505 provides up to a 34 psi pressure depression at 600 gpm. If you only need 27 psi of depression to compensate for the pressure drop through the hose reel, hose and nozzle, you must “bleed off” some of the signal. This is done by adjusting a needle valve (Figure 5) so that some fuel from the high pressure zone at the venturi inlet will flow to the venturi throat. The resulting “mixed” pressure signal (27 psi below system pressure) is then fed back to the control valve as its “command”. When correctly adjusted, the pressures at gauges B and D read the same. This should be checked at no flow and at full system flow rate.

FIGURE 5

• When properly adjusted, the system allows maximum flow rate without danger to the aircraft. If you change to a different hose length or size (diameter), the venturi must be re-adjusted. We recommend that this adjustment be checked at least once a month. In this way, a possible problem with the pressure control system may be detected and maximum safe flow rate is assured.
• If a hose-end pressure control valve (HECV) is mounted on the nozzle, it must be either removed or “blocked out” to adjust the venturi “nozzle pressure”. When finished testing the system, remove the block-out device and re-test the system. (The HECV vent will spray some fuel when next used!) Bear in mind that if the HECV is removed for this test, its own pressure drop will not be compensated for by the venturi.
• Most specifications for fueling equipment now require dual pressure controls, one primary and one secondary or “back up”. This can be achieved by various combinations of inline, bypass and hose-end controls. (We will address this subject in a future Gamgram). A common method has the deadman valve serve as a primary pressure controller with the bypass valve as a secondary controller. Figure 6 shows how this is done. Both valves must not be adjusted to control the same pressure because they will “fight” one another with resulting instability. In any system, the primary regulator controls pressure 5 to 15 psi lower than the secondary. This may be done either by setting the regulators at different pressures, or by sending the regulators differently tuned venturi signals. In this example, the in-line (primary) controller is set at 45 psi, (pressure E). The bypass (secondary) controller is set at 50 psi (pressure B). With this arrangement, if the aircraft valve closes suddenly, the secondary valve backs up the primary valve by opening quickly to relieve pressure on the primary. (Primary pressure may be set as high as 55 psi).

FIGURE 6

Many systems on refueling trucks use air pressure as a reference using an air pressure regulator. It is a good idea to use a locking regulator (See Bulletin 70) to prevent unauthorized or “accidental” adjustment. On a Whittaker regulator, this will be 16 or 25 psi higher than the desired fuel pressure. This difference is called the “bias” pressure. The purpose of this bias pressure is to allow smooth operation of the controller. To better understand this system, think of it this way: The air reference pressure is on one side of a sensing pilot piston with the venturi pressure on the other side with a bias spring. If the total of spring force and venturi pressure is greater than the air reference pressure, the piston moves slightly toward the air side; this results in the control valve closing slightly to reduce system pressure and make the sensing piston return to a balanced position.