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Cockpit Flight Control; PCV with Closed Failure Position

Jan. 16, 2014
Readers asks Our Experts About Safe and Economic Self-Regulating Valves, and Advancements in Cockpit Flight Control Systems

This column is moderated by Béla Lipták, automation and safety consultant and editor of the Instrument and Automation Engineers’ Handbook (IAEH). If you have an automation-related question for this column, write to [email protected].

Q: As I understand it, searchers finally located the final resting place of Air France Flight 447 in 2011 and were also able to recover some of the victims and all of the flight data recorders. It seems that the freezing of the Pitot tubes were indeed the root cause of that accident and later, after the cascade of failure began, there were conflicting control inputs from the copilots until the captain realized (too late) what was happening.

Prior to working here, I used to work in the missile defense industry. We have the ability to accurately hit anything, anywhere with kill vehicles traveling at around 15,000 miles an hour. I believe these technologies can be employed on aircraft as primary or secondary telemetry data sources. I was just wondering if there has been any further discussions or developments on the subject of cockpit flight control systems advancements?

Mark Mason
[email protected]

A: My review indicates that the frozen Pitot tubes played an important role in the Air France Flight 447 tragedy in 2011, and I am also convinced that the Asiana Boeing 777 crash in San Francisco (Korean Air Lines Flight 214) could have been prevented by applying the very basics of automatic safety control, which would have overruled, in one case the actions of the automatic cockpit controls and in the other the copilots’ inaction. As to Pitot tubes, in recent years, there has been some progress in converting to the use of more reliable and redundant speed detectors. On the other hand, the addition of automatic "overrule safety" controls has still not occurred, both because of ignorance and because of cost considerations.

What is meant by "overrule safety"? It refers to the automatic action that overrules all other controls, manual or automatic, and protects the system no matter what. In the processing industries, we have long applied this philosophy by, for example, providing pressure safety valves which cannot be turned off by anything or anybody. Similar "overrule safety" will probably be applied to underwater nuclear reactors, which cool automatically by thermal expansion opening and gravity-loading cooling water, without any valves or pumps. It is time for the transportation industry to also understand and accept automatic "overrule safety" controls that operate just like safety relief valves on boilers or air bags in a cars, in that they cannot be deactivated by anything or anybody.

By the way, the same applies to trains where automatic "overrule safety" controls (ATC) would also be essential. Such systems must automatically limit the maximum speed, based either just on the speed limit at the particular location or can also consider rail curvature, inertia (load on the train), push or pull mode of operation, weather conditions, wind direction, etc. The key is that it is active all the time, and its activation requires no action on the part of the engineer, nor can he overrule it.

Yes, transportation safety technology is available right now. What is missing is the willingness to make the investment needed to add the needed "overrule safety" automation. It is bordering on the ridiculous that, on the one hand, our GPS can measure the location and speed of any vehicle, or that some vendors are considering the use of automatic mini-drones to deliver pizzas, while others feel that automatically limiting the speed of trains or airplanes is too complicated or costly and can be left to bad operating controls and/or to untrained or sleepy engineers and pilots. It is the responsibility of our profession, that of the International Society of Automation, to bring this industry too into the 21st century.

Béla Lipták
[email protected]

A:  Personally, I have found an automatic system that is on by default, but is manually overrideable when needed, to be of most value. However, I also think it really depends on the process under control, because some are just not safely (or even at all) operable in the manual mode. In any case, I think such design decisions should be made on a case-by-case basis and by persons with enough experience/knowledge of the process to reasonably evaluate the pros and cons.

Never having flown a plane myself, I would not be so sure that non-overridable auto speed is the way to go. Some type of warning of the slow speed and that the auto-speed control was only "armed" might be more reasonable.

By the way, I personally really do not like some of the latest air bag safety functions I have come across. For instance, I have been really annoyed after being stymied by the transmission position/brakes interlock when trying to restart an engine that died in traffic. And, although I realize it is not really a fault of the automatic control logic, how about those regularly failing ($900 without installation) BMW passenger seat occupancy sensors and the fact, in my opinion, that such sensors are not atypical?

Al Pawlowski
[email protected]

A: I completely agree that we have the technology to prevent accidents like that. There is a large body of work concerning cockpit automation, under the heading of Situational Awareness. Mica Endsley has done some excellent work. Wikipedia has a good article on Situation Awareness that has lots of references to other work.

Bill Hawkins
[email protected]

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
[email protected]

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
[email protected]

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,
P. [email protected]

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
[email protected]

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 (http://www.ittbarton.com/). However it can withstand a maximum input pressure of 6.9 barg only.

Raj Binney
[email protected]

About the Author

Béla Lipták | Columnist and Control Consultant

Béla Lipták is an automation and safety consultant and editor of the Instrument and Automation Engineers’ Handbook (IAEH).

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