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
bill@iaxs.net

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
Ragab.AbdElFattah@tecnomareegypt.com

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
hvegas@avidsolutionsinc.com

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. Eng.gerald.liu@shaw.ca

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
Debasis.Guha@ranhill-worley.com.my

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
binney4family@internode.on.net

 

 

 

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