Trip and throttle valves and calculating pressure decay over time
Trip and throttle valves and calculating pressure decay over time
(OP)
I've been tasked with doing an initial investigation into a possible new arrangement for a trip and throttle (T&T) valve system for a steam turbine. The T&T valve has a spring which is held open with oil pressurized to 80psi. If the turbine overspeeds, a solenoid valve opens, depressurizing the system and shutting the trip and throttle valve when pressure at the spring reaches 50psi. Normally, each trip and throttle valve has its own solenoid valve located close to the trip and throttle valve. The question that came from a customer is: can we use a single solenoid valve to operate two trip and throttle valves.
The main concern is that by increasing the length of pipe from the solenoid valve to the T&T valve, we could delay the closing of the T&T valve because the pressure wave would take longer to get there. Some engineers I've talked to don't think this would be a problem because the pressure wave will travel at the speed of sound, adding only a negligible delay to the closing of the valve (which normally takes around .450 seconds to close from the time the governor sends the overspeed signal and the spring slams the valve shut). However, another engineer recommends calculating the pressure change as a function of time at different points in the system.
So my main questions are:
1. Is it possible/necessary to calculate the pressure decay over time due to the rapid opening of a solenoid valve?
2. Are there any other concerns with this design that I may be overlooking?
Thanks!
The main concern is that by increasing the length of pipe from the solenoid valve to the T&T valve, we could delay the closing of the T&T valve because the pressure wave would take longer to get there. Some engineers I've talked to don't think this would be a problem because the pressure wave will travel at the speed of sound, adding only a negligible delay to the closing of the valve (which normally takes around .450 seconds to close from the time the governor sends the overspeed signal and the spring slams the valve shut). However, another engineer recommends calculating the pressure change as a function of time at different points in the system.
So my main questions are:
1. Is it possible/necessary to calculate the pressure decay over time due to the rapid opening of a solenoid valve?
2. Are there any other concerns with this design that I may be overlooking?
Thanks!





RE: Trip and throttle valves and calculating pressure decay over time
2) You probably would not want to increase the depressurization time as depressuring with only one valve might.
Does the use of two valves have anything to do with increasing the reliability, or hastening the pace of depressurization of the trip system?
3.) Don't forget the first law of reverse engineering... If it ain't broke, don't fix it.
I hate Windowz 8!!!!
RE: Trip and throttle valves and calculating pressure decay over time
There is a potential issue in terms of increased fluid mass which needs to move if you move the solenoid valve further away, but without a diagram and dimensions I'm struggling to see any positives from this proposed arrangement.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Trip and throttle valves and calculating pressure decay over time
I hate Windowz 8!!!!
RE: Trip and throttle valves and calculating pressure decay over time
So their questions are
1. Will a single solenoid rack be able to cause both T&T valves to trip? (I don't see any reason why it wouldn't)
2. How long will it take for each valve to trip?
The preliminary position of the engineers I've talked to is that an additional solenoid rack would be unnecessary, but we may need to provide some calculations to answer the above questions. Big Inch, you mentioned it is possible to calculate the pressure decay over time. Could you point me in the right direction? Could an approximate analytical solution be found, or would this require some CFD modeling? I've been looking into the Joukowsky equation, but I'm not sure if that is the right approach and I don't see how I can calculate pressure as a function of time.
Thanks!