I would like to know from the design point of view. If we have ESD ZV, is it possible or recommended to make the valve closes with time delay. We have 5 valves 3 of them closes with 10 to 20 seconds delay. I suspect that there is time delay set in the PLC logic. Does this sound logical when we have triconex ESD system?
Not sure what the last two letters mean but if ESD stands for "Emergency Shutdown Valve" and you have a twenty second delay I would be worried.
Our Australian Gas Code as well as "Good Engineering Practice" would expect a closure of between 2 to 3 seconds maximum.
I checked with our engineering and they confirmed that their should not be any delay. I did check the power suplly coming to the selnoid and it became energized the moment we press the close push button. We opened then the pilot and did the necessary cleaning ,but we still have the problem.
If your SOV is de-energizing and the internals are OK and the valve actuator is still taking time to close then it could be one of the following;
1. Actuator is sticking. O-ring has become too dry (use some vaseline).
2. Actuator is leaking if double acting or the Spring has lost it's tension if single acting.
3. With regards to the later, make sure no one painted over the breather hole on the spring side of the actuator ;-)..not kidding.
4. Could be the valve (wetted part) sticking - that's why the PVST (Partial Valve Stroke Test) now comes into play..pull the valve and check in the workshop.
My copy of API RP 14C is the Seventh Edition, March 2001.
The paragraph that mentions the 45 second requirement is in Appendix C - Support Systems. The paragraph begins on page 65, C.2.1.4 Supply and Response. The last sentence on page 65 continues on page 66 with the 45 second requirement.
"The time it takes for any safety device (e.g. PSH, BSL, ESD station, etc.) to effect component or platform shutdown should not exceed4 5 seconds. To achieve this response, consideration should be given to pneumatic line sizes, safety device bleed port size and the use of auxiliary quick bleed devices. Pneumatic lines that supply and bleed should be sized for optimum bleed conditions. Because of volume and flow characteristics a line that is either too large or too small will require excessive time to bleed."
The next sentence applies an obsolete API Recommended Practice 550 to be used as a guide when designing instrument and control systems. Instead the paragraph should reference 551-554, etc.
>Not sure what the last two letters mean but if ESD stands
>for "Emergency Shutdown Valve" and you have a twenty second
>delay I would be worried.
>Our Australian Gas Code as well as "Good Engineering
>Practice" would expect a closure of between 2 to 3 seconds
there is no need to slam a valve shut if the machine or process the valve interrupts is a consequence of a shut down, but none of its monitored parameters is the root cause.
When I provided ESDs (then called Emergency ShutDown systems) two trip-valves were often provided... one used for full closure the other provided a ramped closure.
For example, immediate closure of a turbine's steam supply for loss of lube-oil is certainly warranted, while ramping it closed for, say, overtemperature.
The intent is to reduce the impact on ancillary services like steam boilers, hydraulics, etc.
If you would like a copy of the DO's and DON'T's
associated with Shut-Down System Design, contact me off-line.
Phil Corso (cepsicon [at] aol.com)
The answer is YES .. this could be logical. and the answer could also be NO .. this is not logical.
The best one who can answer and determine this is you!!
You need to study your process, piping grid layout, and ancillary equipment needs for Emergency ShutDown.
If you can read PLC Programmes, and knowledge of ESD, you could be able to analyze and figure out how your ESD should work.
You need to do some alanysis, and share your findings with us.. I am sure that many of us will be able to help you with your query if facts and details about your system are provided.
Adding a bit to what Ashraf said:
First is it the valve or the ESD causing the delay?
It is surely very easy to find out, just 'pull the wire' and watch the valve - as the ESD should fail safe that should be safe, but of course check first!
Assuming it is not the ESD but the valve, then:
Check the valve spec and the installation. And if relevant the vent piping on the actuator - if the pipe is too small or long to vent quickly.
I would very much hope it is not the ESD but if it is then such a time delay would have to be programmed for a reason. Find it.
Further adding to Francis regarding the vent pipework, if you are able to hear it venting throughout the 20second closing time this would effectively prove that the vent pipework is too small/too long.
Hope this helps.
It has been confirmed by our engineering that the PLC logic used cannot be other than yes or no or in another way 1 or zero as you know. I need to know though that can modern PLC configured to to have some delay if we wish.
(Any way the problem was solved as lot of dirt was found in the pilot valve as probably the LPG loading rack was out of service for a long time and that cause the dirt to accumulate)
Thanks to every one participated