Maximum flow through 100% open valve

[curtis2004]

Hi Everyone,

I'm having a brain fart and can't remember how to calculate max. flow through a valve for PSV sizing purposes. I have a superheated steam line with pressure reducing valve:
Inlet and Outlet Line size: 3"
Valve size: 3"
Pinlet = 190 psig
Poutlet= 73 psig
Tinlet = 275C
Flow = 700 #/hr.
My downstream piping pressure rating is 150 psig. So I need to calculate flow through this valve fully open and deduct 700#/hr flow in order to find out what required capacity of PSV. Am I correct? How to calculate this maximum flow?

Thanks,
Curtis

 

[curtis2004]

in addition ...
The valve's sizing sheet doesn't have selected valve's Cv. It only lists required Cv at given flow. Selected valve is T-Globe valve.
Do we need Cv at 100% open position to calculate max flow?

Thanks,
Curtis

 

[LittleInch]

I think you need CV at 100% open.

you can only remove the 700 lbs/hr if the flow is somehow 100% guaranteed while your regulating valve goes 100% open?

Even if its just running at say 300lb/hr rather than isolated, this will impact on your relief valve capacity

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[MortenA]

Agree 100% with Littleinch - you need the capacity of this valve - unless its a block valve type such as a ball or a gate valve. In this case you can assign somewhere close to an infinite Cv (can be hard to get your head around - but the point is that the resistance in a fully open ball (or gate valve) is so close to zero that you have to look somewhere elso to determine your require relief capacity e.g. total pressure loss in inlet piping etc.

If you can get the Cv then its easy: Cv=Q*sqrt(SG/dP) for a liquid. If you are dealing with a gas its a little more complicated...

Best regards, Morten

 

[racookpe1978]

So, his required reducing flow is met by an orifice when the valve is 100% open, then he gets less flow as the valve is throttled?
But the max pressure in the remainder of the line is the 70 psig, right? So the LP relief setting has to be based on the desired 7- psig, but is a separate relief valve required for the higher pressure part of the line since the orifice cannot "fail" ?

 

[curtis2004]

Hi there,

Just to give you more details of my system. I have auxiliary system, which required a constant flow of 700# steam at 73 psig. This flow is guaranteed and consumed by steam ejectors. This system is fed from another steam source with 190 psig pressure a through pressure control valve (T-globe type valve).
This auxiliary system has a pressure vessels and components with MAWP of 150 psig and supplied with a PSV. I'm trying to get answers from supplier about PSV valve supplied with the system. However, it seems she doesn't have a clue why they supplied this PSV and how do they sized it.
I think, think this PSV shall be sized for the T-globe fail 100% open. In case is T-globe valve fails and opens at 100%, PSV shall be able to remove flow and keep a pressure under 150 psig.
I think, there shall be a formulae for calculating a flow through a valve with 100% open Cv, P1 = 190 psig and P2 = 150 psig, which gives us bases for sizing of PSV. However, I have checked T-globe valve sizing sheet and it doesn't have Cv100% value for the valve selected. I guess, I have to go back to the T-globe valve supplier and get Cv100% for the valve.
Do you know what formulae shall I use for calculation of flow through 100% open valve?

Thanks,
Curtis

 

[MortenA]

I overlooked that it was steam - check this page for directions:

http://www.engineeringtoolbox.com/flow-coefficients-d_277.html

Remember that your relieving pressure is 1.1*SP if the valve is an API 520 valve

 

[LittleInch]

Also if your downstream system is essentially open to atmosphere or some other system without other valves in the way and operates at 73 psi, then maybe even with the prv fully open, you won't reach 150 psi??

Worth finding out what the flow through your d/s system is at 150 psi before you go and put in a relief valve that might not be required.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

Careful there - get help from someone who does this work routinely. Also remember a PSV is a high integrity pressure protection device - you may need to provide volumes of documentation to show how sure you are of this guaranteed flow of 700lb/hr of steam from this PCV to the auxiliary system under all operating modes and failure modes.