Orifice Plate Permanent Pipe Losses

[jnb14495]

I am designing an orifice plate to reduce the pressure in a system. I know how to use orifice plates to measure flow via differential pressure, but I do not know how to calculate the losses in head pressure in a system due to this constriction. Anybody know of some resources?

Here are my givens.

Fluid- Water
Pressure- High side 2800 psi
Pipe diameter- 3.438 in
Desired pressure downstream- 950 psi
Find Orifice diameter

Thanks for your help

jnb

 

[Artisi]

Sounds like a college exercise - a search of your text books should find it for you.

 

[dcasto]

search an RO.

 

[katmar]

The two resources I would use for this are the Crane 410 manual, and the article by William B Hooper (The 2K Method man) in Chemical Engineering, Nov 7, 1988, pg 89-92

You have not specified a flowrate.


Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[JLSeagull]

Miller and Spink address the permanent loss for flow elements. Consider using the pipe tap calculations for permanent loss.

 

[zdas04]

About 10 years ago a sub-committee of the API 14.3 Gas Measurement Committee developed a correlation for permanant pressure drop that was published in the April, 2000 version of API 14.3 Part 2 (section 2.4.5.1). This is presented as a table where permanant pressure drop varies with beta ratio (for a 0.2 beta you lose 95% of the dP, for 0.75 beta it drops to 45%).

These numbers are for an API 14.3 plate. If you increase the thickness of the restriction, the permanant pressure drop increases until it approaches 100% for all choke sizes at about 3 pipe diameters.

David

 

[dcasto]

I think he's looking for RO with a delta p of 2800-950 = 1850 psi.

 

[hacksaw]

Hardly a text book problem, but you don't generally use an orifice plate for such applications.

if this is a pump by pass, the pump manufacturer can sort you out.

 

[CJKruger]

jnb14495, three different approaches are commonly used for an RO:

Approach 1: Use equations for "pipe-taps".

Approach 2: Use equations for "flange taps" (or close up taps) and correct dPcloseup to get to dPpermanent.

Approach 3: Use K value equations (from Hooper for example).

I have always used Approach 1 or 2 and agree with JLSeagull that Spink and Miller is the best reference for this.

 

[jnb14495]

Quick question--- What does RO stand for?

Thanks a lot for helping me out with this. The first post in response made me feel like I really missed something in college. Orifice plates are typically designed to measure flow and typically the only place of concern with pressure is before the plate and within the vena contracta that the plate produces. They are not typically used in the fashion that I described, but can be. I am part of a start up engineering team and we need to drop the pressure to one of the end users during a system flush.

 

[proinwv2]

At that DP I would also look at the strain and deflection of the plate.

Paul

http://www.ostand.com

 

[25362]

I believe that RO = reducing orifice

 

[Gator]

Or maybe Restriction Orifice? By definition, any orifice is "reducing", it seems to me that "restricting" better defines its purpose but maybe that's just semantics.

 

[25362]

I fully agree. See:

http://www.tercom.it/index_file/Page681.htm

http://www.int-instrument.it/Inglese/Pagina18.htm

 

[BRIS]

simple answer is that the orifice (same as a valve) causes an increased velocity through the constriction. This velocity energy is not recovered downstream. The head loss is equal to the energy loss.

Converts potential energy to kinetic energy and destroys the kinetic energy.

calculate the area of flow through the orifice (= orifice area 8 contraction coefficient (0.65 say for blunt edge) calculate the velocity through the contracted orifice.

Head loss = (Velocity through the orifice - downstream pipe velocity) ^2/2g (SI units)

 

[BRIS]

simple answer is that the orifice (same as a valve) causes an increased velocity through the constriction. This velocity energy is not recovered downstream. The head loss is equal to the energy loss.

Converts potential energy to kinetic energy and destroys the kinetic energy.

calculate the area of flow through the orifice (= orifice area x contraction coefficient (0.65 say for blunt edge) calculate the velocity through the contracted orifice.

Head loss = (Velocity through the orifice - downstream pipe velocity) ^2/2g (SI units)

 

[toothless]

How can you design an RO to control downstream pressure unless you can perfectly control the flow rate? An RO will produce virtually no pressure drop at low flow (low velocities). This is why the control valve was developed.

 

[davefitz]

The use of a fixed orifice for a high DP RO with water has 3 major problems:

a) only works correctly for one single flowrate

b) severe erosion downstream of the orifice and at orifice is likely- in such applications ceramic orifices have been used, but a much better fixed restriction is to use a small diameter stainless steel capilary tube.

c) prediction of flow will be tough if flashing or cavitation occurs. If teh pressure in the vena contracta drops below saturation pressure, steam bubbles will form and choke of the flow, then collapse at soundpseed after pressure recovery and severely erode via cavitation. Again, a SS capilary tube obviates this issue.

If the process conditons are expected to cahnge ( flowrte, fluid temp required pressure), then one should use an anticavitation control valve.

 

[dcasto]

I concur about damage. Since we don't know all the facts, I just led him to a water trough.

I've seen similar problems, the up and downstream pressures are fixed by other control systems. If you needed to maintain a minimum flow on a pump with the given conditions, you size the orifice to do maintain the m inimum flow. Yes it appears to be a waste of energy, but, it depends. Small injection systems where min flow is 5 gpm, that wouldn't be much of a loss and the capital for an RO is almost zero compared to an elaborate valve.

Another place is where you have a blow down systen durin an upset. Example: There is a 2" ball valve that vents to VRU to get the material out of the system. We didn't want to flow more than 15,000 lbs/hr. Plug in the up/down stream pressures, max flow, bingo a .375 plate. Its only used in an emergency, so worrying about errosion isn't a concern. we had this type arrangement all over the plant.

 

[BRIS]

True you can only size the orifice plate to give a specific pressure drop at a specific flow rate. The flow rate is not stated so the problem is unsolvable. Cavitation needs to be checked and is overcome by providing a series of orifice plates. [We have several hundred orifice plates providing pressure drop at well pumps (temporary installations to limit pump discharge until well field draw down reaches design draw down) and in irrigation distribution systems to limit flow] Bernoulli’s equation and the assumption that there is no kinetic energy recovery (velocity head recovery) between the vena contracta and downstream flow gives the solution for pressure drop and potential cavitation.