Cockpit Flight Control; PCV with Closed Failure Position

Readers asks Our Experts About Safe and Economic Self-Regulating Valves, and Advancements in Cockpit Flight Control Systems

By Bela Liptak

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Bill Hawkins

Q: We are going to install self-regulating pressure regulating valves (two valves in series) at the inlet of a new air cooler skid to reduce the upstream pressure from 125 barg to 5 barg (from 125 barg to 50 barg and then from 50 barg to 5 barg). The pipe size is 3 ins. The required CV is 5.86 calculated on maximum flow rate of 40,250 Kg/hr and DP of 55 barg. The fluid is water.

Due to safety and economic issues, the process engineer asked us to provide these two self-regulating valves with fail-closed positions! Are there such valves that can regulate the downstream pressure and fail closed? We have no electronic control system in the plant, so we have to install self-regulating valves.

Could you please advise?

Ragab Abdel Fattah

A:  Why would you waste all that good pumping energy that this 40,250 Kg/hr (~ 190 GPM) water stream contains? If you need water at 5 barg (~ 73 psig), it makes no sense to obtain it from a 125-barg (~ 1,800 psig) source! So my first reaction is to get a new process engineer! If you do as your process engineer suggested, the vibration and cavitation will destroy the valve in no time at all, even if you pick the most tracherous flow path designs (multi-port, multi-path, ‟Swiss cheese," cage, what have you). If the flow was relatively constant, you could consider restriction orifices or chokes, but even they would not last long, but at least they are cheap.

As to self-contained pressure regulating valves with closed failure positions for such an application, there is no such thing on the market (fortunately).

Béla Lipták

A: One option is to install a regular fail-closed pneumatic control valve with a pneumatic controller. This arrangement could probably be sized to handle the letdown in one step and would fail closed on loss of air.

Hunter Vegas, PE

A: Dropping pressure with a severe service control valve (not a self-regulating pressure valve) from 125 barg (almost 1,875 psig) to 50 barg (almost 750 psig); i.e. 60% drop, is difficult because that requires a trim loss coefficient (k-factor) of about 16. I don’t think any self-regulating pressure valve can do that. The loss coefficient of a drilled hole is typically 1.5 at most and, therefore, you would need about 16/1.5 or 10 stages of drilled holes at least (in series) to drop from 125 barg to 50 barg.

At present, with the best of technology nowadays, control valves with 7 drilled-hole cages is the maximum number of stages that any control valve manufacturer can implement, because the number of stages for drilled-hole cages is limited by the size of the valve flanges. Furthermore, the individual resistances of the stages in series within a control valve do not add up in arithmetic progression to form the overall trim loss coefficient, but in geometric progression, approaching an asymptotic limit. In other words, 10 stages of drilled-hole cages in series do not give us an overall loss coefficient (or resistance) of 10x1.5 or 15, but a number substantially less than 15, such as 11 or 12, depending on the size of the hole in each stage.

Dropping the pressure from 50 barg (almost 750 psig) to 5 barg (almost 75 psig) is even more difficult because that is a 90% drop and is even further away from the critical pressure drop limit (which is about 50%, depending on the nature of your process medium).

I had one application that required fixing the problem of a steam pressure drop from 600 psig to 50 psig, where the vibrations of the valves owing to shock waves in the valve exit eventually rendered the valves permanently shut. To give you an idea of proper control engineering, to vent steam from 50 psig to atmosphere, a control valve with 36 stages of resistance is required (not in drilled-hole cages, but in right-angle turns in discs) to cope with noise regulations. So I don’t think any "ordinary" self-regulating pressure valve can drop the pressure from 50 barg to 5 barg. ("Ordinary" here means valves with drilled-hole cages).

Regarding the failure mode, you can specify fail-closed and that can be implemented. But you have to specify exactly what medium will fail; i.e. power failure or signal failure. In the case of a self-regulating pressure valve, power failure is the upstream process pressure loss, while signal failure is the impulse line pressure loss (which can also be due to upstream process pressure loss or tube burst). With accumulators (air or hydraulic, depending on your requirements), any valve can be designed to fail closed or fail open. However, I do not know any self-regulating pressure valve(s) having fail-closed functionality yet. I would not say they do not exist.

Please discuss this further with your process engineer. I was also a senior instrument engineer in one stage of my career.

I hope comments help.

Gerald Liu,

A: Self-acting pressure regulators can achieve a specific fail position based on spring/diaphragm. You can specify in the datasheet failure mode and direction. If you do not specify, it will assume default position. In short, you can implement a process requirement.

Debasis Guha

A: Self-regulating pressure regulators are mechanical devices and, hence, do not have a fail-safe feature. The best you can do to address the particular concern is to install safety relief valves (SRV or PSV) immediately downstream of the pressure regulator. Hence, you can install an SRV with a setpoint of 51 barg after the first regulator and a second SRV with a setpoint of 5.5 barg after the second regulator.

A new device is available from ITTBarton ( However it can withstand a maximum input pressure of 6.9 barg only.

Raj Binney




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  • The NTSB published their report of the recent Asiana flight that crashed in San Francisco.

    I find the contrast between the views of the NTSB and your views on the subject of using automation on the flight deck of an airliner interesting. According to the NTSB, the pilots didn't understand the automation in place, and that they mistakenly believed that it should have done something to arrest their high descent rate. In fact, there is such a mode in the airliner autopilot systems, but that's not what they had selected.

    Allow me to share some of my experiences with the human side of automation:

    I have been employed at a large water and sewer utility for more than 28 years. In the mid 1980s we were early adopters of automation. We got started with those lovely old PM550 controllers from Texas Instruments. The first thing we did was to replace those cam stacks and microswitches for sequencing through a filter backwash.

    The new backwash system was very effective. The operator would push a button and with great reliability, a backwash sequence would happen. They could forget about the details, the interlocks, the permissives, and even where the valves, and pump controls were. And they did.

    In just a few years, most of the operators had forgotten how a backwash worked. Only the senior plant operator, the controls engineer, and the plant superintendent remembered why things were done the way that they were. And they got more and more grandiose with their designs.

    We upgraded the control system and then the superintendent went wild. We had backwash schemes for energy savings, for water savings, for speed, for deep cleaning, and other various permutations and combinations. And then he retired. The controls engineer moved in to a new job and pretty soon we had voodoo with a platform that was rapidly becoming more and more obsolete. We were scared to upgrade it (but we are doing just that).

    Complexity and the maintenance, management, and operations of such complexity often often forgotten in the design of complex systems.

    You make the point that automation could stop people from making stupid mistakes like not maintaining speed, or turning too sharply, or not shutting the plant down properly. And perhaps you're right --but it leads to more and more complexity and confusion.

    That's what happened with Air France 447. The airliner Pitot tubes iced up at an altitude where that was not supposed to be possible. The controls reverted to Alternate law because the automation had no contingencies to handle three wildly different air speed indications at an altitude where the operating range between the wing stall speed and the compressor stall speed can be as little as 12 knots.

    Had the pilots been more experienced with manual controls they would have known in a heartbeat what to do. But they had forgotten.

    I fly small airplanes on instruments. Manually. I have a constant feel for what my airplane is doing. Yes, my instrument approaches are sloppier than someone's three axis autopilot with auto throttles. But I KNOW where I am and I know what is supposed to come next. And because I do this for fun, I know better than to fly when I'm tired, stressed out, or ill.

    I think that until the instruments, automation, and controls get extremely reliable, we'll still need to keep the humans in the loop. Sooner or later that instrumentation or automation will fail. And then, with so little experience working without automation, the human won't know what to do either. That's the lesson I take home from Three Mile Island, from Fukushima, from Air France 447, and so many other disasters.

    Thank-you for all the good work you've done, Mr. Liptak. You remain a giant in this industry to our engineering staff at the Washington Suburban Sanitary Commission.

    Jake Brodsky


  • Thanks Jake, You summed up the present cultural-state of the human-machine relationship perfectly and I fully agree that our society is in a state of confusion. The new generation of „button pushers” are growing up believing that it is good enough if Google and Wikipedia is smart, they do not nee to be . Having said that, we must not let programmers run wild. Our role, the role of control enginers is esential, because we know how to keep a pipe straight (BP) or how to build a nuclear power plant under water so that it needs no man made energy to be absolutely safe. If our knowledge is properly applied, automation can make industry safer by preventing panicked or ignorant operators (or terrorists) from doing unsafe things. Overrule safety controls (OSC) are like red light or lift gates on street corners because on the one hand they do not prevent the driver to say visiting his mother-in-law, but they do protect both him and others. So what does this mean for Air France 447? It means only two things: 1. It means that bad sensors should not be used. Pitot tubes can freeze up, static pressure based altimeters can give false information when air density changes (cold fronts, etc.) So forget such ancient sensor and use radar or GPS. 2. OTC must be on all the time and must not depend on what the pilot believes or what he selects. The pilot, - just like the person crossing the street - must not be allowed to turn off the “red light”. OSC must be on all the time and prevent the pilot from doing stupid things. Best Regards, Béla


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