Trip and throttle valves and calculating pressure decay over time

[DanielWW]

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!

 

[BigInch]

1) It is possible, but I agree that unless the pipe is several miles long, it probably won't make much difference in the time needed for activation. I would think that it is not so much the time for pressure waves to travel as it would be time to depressurize the oil after arrival of the pressure wave trigger and activation of the solenoid, which would probably also be rather fast.

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!!!!

 

[LittleInch]

My question would be why does the client want to do this?. A solenoid valve is peanuts compared to the turbine and if your signal is electric or fluid powered from the governor then it can surely activate two solenoid at the same 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

 

[BigInch]

I agree and that's precisely why I warned about not fixing this. I also struggle to see any real potential benefit.

I hate Windowz 8!!!!

 

[DanielWW]

My apologies, I had the situation mixed up. Usually we use one solenoid rack with four solenoid valves to control two trip and throttle valves. Because of the geometry of the site, one T&T valve is to be located at a higher elevation than the other T&T valve. The customer is concerned that they may need an additional solenoid rack to actuate the T&T valve at higher elevation. Because they may have some special requirements for these solenoid racks (I'm not sure exactly what the details are) a single rack may cost up to $100K.

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!