valve differential pressure? 1

[thrusterpump]

Hi folks,

I need to size a valve that will have a short pipe downstream of the valve discharging to an open resevoir and I need to provide the max. differential pressure. Is this dP just the upstream pressure-atmospheric pressure? Please correct me if I'm wrong and thanks in advance.

 

[itdepends]

It's the difference between the max pressure upstream (shutoff pressure of pump + max static head) and atmospheric. So yes- you're basically correct (just make sure you use the right upstream pressure).

 

[terje61]

Hello Thrusterpump,

Be carefull:
When you are reducing the pressure to atmospheric pressure a valve is often choking. This means that the outlet pressure at the valve is no longer depending on the outlet conditions, but only on the inlet conditions. The actual pressure drop over the valve will therefore be less than P1-Patm. And several shock waves will occur between the outlet flange (P2) and Patm. The remaining pressure drops can appear in the pipe, in the outlet of the pipe and even beyond the pipe.

Regards,
Terje

 

[JimCasey]

Terje brings up a good point. His remarks are particularly applicable to gas service. SInce you are discharging to a reservoir, I interpret it as a liquid flow. Liquids have a critical pressure drop governed by the pressure recovery coefficient (Fl) and the vapor pressure of the liquid. In any case once the critical pressure drop (DPcrit) is exceeded, the downstream pressure becomes independent of flowrate and the flow is said to be choked.

>Standard rant #104<
Choked flow does NOT mean that the valve cannot pass any more flow. Opening the valve more will, in all but extremely rare circumstances, result in additional flow. Increasing the upstream pressure at a given valve opening also will result in additional flow. Really the prime consequence of choked flow to an applications engineer is that a different equation governs the calculation.
Flashing flows or cavitating flows are generally choked.

 

[tsenthil]

HI Jim,

Thanks for the post.. Can you explain how additional flow will happen in a choked condition by opening the valve?

Thanks

Senthil

 

[terje61]

At non-choking conditions the flow depends on:
- Upstream pressure
- Downstream pressure
- Medium density at flowing conditions
- Valve opening

For choking applications the flow however depends on:
- Upstream pressure
- Medium density at flowing conditions
- Valve opening

Conclusion:
When you open a valve in either situation the flow will increase.
As mentioned by Jim: opening the valve more will result in additional flow. Be carefull this is not an additional increase in flow. 10% more Cv = 10% more flow.

 

[gerhardl]

Note that you do not only have to size the valve, but actually also to select the correct valve for the application.

For lower pressures and slow flow a simple butterfly valve may be suficient.

Be careful if the pressure is not in the lower range and if you want to regulate the flow, and check that the valve is constructed not only for the pressure range but also for the flow speed.

As you do not give the actual data you might well have a classical case where a special designed regulating valve must be selected to avoid or overcome cavitation, or you might have to have pressure reduction in several stages, discharge in special design to air, or air inlet etc. all to the same purpose of controlling the possible cavitation.

Direct discharge to air after the valve (without the short pipe) could be a solution.

If only on/off application a good, stable, preferably trunnion mounted ball valve will withstand higher water speeds than other valves and could be mounted inline.

 

[thrusterpump]

Thank you all for the input. The following is some more application info as I continue to search for a suitable valve:

Pump s/o pressure = 1200 psi
Clean water @ 70F
5" nominal pipe through 5" venturi
Subject valve discharging to atmosphere through 2'length pipe. Ideal valve would have no leakage when closed and have good flow control characteristics.

Any thoughts to brand/type ?

Thanks again all

 

[Joesteam]

Now that you have it down to nuts and bolts, do you know the required flow through the valve? AND what type of control will you use or do you just want an open/closed. If you want the maximum DP through the valve, then just use a full port type of valve. There are some great butterfly valves with bubble tight shut-off for liquid service, however it will be tough to find them in 5" (you may have to go to 6")

WKM brand:

http://www.c-a-m.com/content/products/product_detail.cfm?pid=2958

Joe Lambert

http://www.control-associates.com/

 

[Scotsinst]

Is this valve a low-flow pump protection valve? With inlet of 1200psig and discharge approximately 0 psig I would recommend a multi-step valve like a Fisher Cavitrol III 3-stage, Masoneilan 14200 series, Valtek tiger tooth or CCI drag valve with Class V or VI shutoff. I recommend you contact vendors for their recommendation.

 

[thrusterpump]

we want to control the flow rate from 90-1200 gpm for controlling the rate at which the resevoir fills. Pneumatic actuation is prefered. I have noticed 5" butterfly valves are less common.

 

[gerhardl]

Converting to European standards I have briefly looked into your problem.

Converting I find
Given pump pressure about 83 bar
Min flow 5,6 l/s- 20,6m3/h
Max flow 75,4l/s - 271m3/h
5" valve (Nominal diameter 125mm) giving teoretical flow 0,46 to 6,1m/s.
Water 20 deg C.
All this is well within 'normal' for regulating valves; there is however a very large 'BUT':

The pressure drop of 82,3 bar directly, one step, will lead to all problems and situations pointed to by terje61 and myself, with immidiate cavitation, shocks, noice and shutterings even in the short pipeline after the valve.

The best solution will be to try to find a special regulating cage-type or needletype valve (butterflyvalves will cavitate!) and discharge immidiate to free air after the valve, no pipeline at all.

Even at that it could be difficult to find a valve that could contain the large pressure drop. I would suggest in practice between half and a third of the given pressure drop as a maximum.

The best solution would be to find a way to reduce incoming pressure before the regulating valve. (What gives the necessity for the high pressure? Fall highth? Inline reducing stations higher up possible?).

You could of course also/instead try to mount orifice plates before or after the reducing outlet valve. Because of the wide range of flow regulating I would not recommend this as the solution. You will need a complete different set of plates/restriction for the lower flow and the upper flow range.

Another possible solution would be an on/off valve, given a set, constant flow. You could then combine with a set of reducing orifice plates before the valve, and on/off direct float actuation (solutions for this exists) of the valve for reservoir max/min level. For thigtness and floatvalve type you wil also here probably have problems finding a valve above 40 bar (European PN40).

Have a look at

http://www.erhard.de

(Tyco waterworks) - downloads.