Calculating pressure drop across Valve

[Mavericks7]

Hi, I have a few questions and wonder if anyone can help me. I need some way to work out the pressure drop across a oriface plate or sluice valve at the point the valve is nearly closed, i.e where the first distinction between the background sound, and the sound induced by what I guess is the sonic velocity of the fluid (through the pressure drop through valve closure) is measured. Unfortunatley it is very difficult to measure the pressure the other side of the valve (outlet), so I need some kind of Physical law/equation (for water) that can derive this, that also considers the mains/pipe pressure (inlet pressure) as a variable. If anyone can provide any insight or maybe point me to where I can find the releavnt literature, it would be much appreciated. Thankyou.

 

[gerhardl]

For water you can calculate according to normal flow equations either pressure drop (delta p), flow coefficient (Kv European Cv American) or flow (Q) if the other two are known.

Thus the only way if some of the necessary parameters are missing is either to establish those missing or making approximations.

 

[MortenA]

Why is the pressure difficult to measure?

You cant really "calculate" the dp - since you (intrically) assume == no flow. So as a matter of fact (trivial i know) dP=P(upstrem)-P(downstream).

Flow and pressure are two sides of the same coin. You cant separate them.

 

[sailoday28]

1 What are the up and downstream conditions of the orifice plate as it starts to close?
2 Is this open channel flow?

 

[gerhardl]

Clarifying, and presumed known:

Kv or Cv are the given (measured) flow of a valve at given parameters, with given dimensions (m3/h at a pressure drop of 1 bar for Kv) (eg NOT a dimensionless factor).

If Kv and Q both are known for the valve, you can find necessary delta P (driving force) for liquids through the valve.

Delta P = (greek ro) x (Q/Kv)2 /1000, equals approximately (Q/Kv)x(Q/Kv) for water

(greek ro = density of medium upstream of valve kg/m3 ca 1.000)

 

[lugnuts67]

What type of valve is it and can the manufacturer supply you with a table on the Cv of that valve by % open or degrees of rotation? Then you could plug it into the simplified sizing equation.

 

[BRIS]

Why? It would be useful to know your objective.

If the valve is producing a large amount of noise, as your post suggests, then it is likely to be operating in a choked (cavitating) condition. The downstream pressure is vapor pressure. Flow is independent of downstream head.

If flow is not choked then you wil need to either know Q to calculate delta P or delta p to calculate Q. i.e you need to know 2 of the three variables.

 

[davefitz]

In the case where you have choked flow, only the inlet P, inlet P, inlet molecular wt,gas ratio of spec heats, and the orifice area and geometry define the flowrate; the downstream pressure is irrelevant.

Very close to the outlet of the valve/ orifice, there will be a series of shock waves, and the pressure will discontinuously decrease across these . There are lab techniques to visualize these ( schlieren photography, laser anemometry), but I am not sure the results are repeatable nor can you determime the exact pressure drop across each distinct shock , but I could be wrong. See you local engineering school's professor of compressible flow.

 

[sailoday28]

davefitz (Mechanical) 7 Jun 07 8:10
In the case where you have choked flow, only the inlet P, inlet P, inlet molecular wt,gas ratio of spec heats, and the orifice area and geometry define the flowrate; the downstream pressure is irrelevant.

The flow will be defined if given 2 of the following 3 upstream variables:

STAGNATION pressure, denity or temperature.

 

[davefitz]

sailoday:

as far as I know, there is still a need to know the ratio of specific heats . The geometry of the hole also effects the discharge coeficient, epescially for "square edged" orifices undergoing choked flow ( ie ratio of thickness to dia meter). For a valve, the geometry near the area minimum effects the onset of "oblique shock waves".

 

[Arrayproducts]

Here is something that i had put together,see if this helps.This should be applicable to find the pressure drop accross the Gate/Sulice valve too.
The pressure loss (or pressure drop) in a pipe, tube or duct can be expressed with the D'Arcy-Weisbach equation.
?p = ? (l / dh) (? v2 / 2)
Where
?p = pressure loss (Pa, N/m2) – Multiply by 0.000145 to arrive at psi.
? = D'Arcy-Weisbach friction coefficient – For example use 0.019
l = length of duct or pipe (m) – Use length from flange end to flange end of valve
dh = hydraulic diameter (m) – Valve bore size
? = density (kg/m3) – Density of flow media
v = velocity (m/s, ft/min) – Velocity of flow media

Regards,
Suresh

 

[Arrayproducts]

I just noticed that some of the characters are not displayed properly in my previous message.So i modified it:

Pd = L (l / dh) (d v^2 / 2)
Where
Pd = pressure loss (Pa, N/m2) – Multiply by 0.000145 to arrive at psi.
L = D'Arcy-Weisbach friction coefficient – For example use 0.019
l = length of duct or pipe (m) – Use length from flange end to flange end of valve
dh = hydraulic diameter (m) – Valve bore size
d = density (kg/m3) – Density of flow media
v = velocity (m/s, ft/min) – Velocity of flow media


Regards,
Suresh

http://www.arrayproducts.com