Restriction Orifice

[MeEn]

Hi Guys,
I know that restiriction orifice control flow, but do you know how they control pressure?

Is it possible to use RO as pressure regulator? and if so, how is it done?

Bye the way, is any web sites on the internet talking about how RO works? Please let me know if any.


thank you very much
MeEn

 

[MJCronin]

MeEn,

ROs are sometimes used as part of a recirc/distribution loop in a system with several "users". For example, they would be installed in a hot glycol distribution system.

The purpose of the RO is to allow the pump to operate within a reasonable part of it's operating curve and to ensure a pressure drop between the supply and return part of the loop.

AN EXAMPLE:

From the circ pump discharge, small taps would go from the main pipe to each user. At the end of the distribution chain, a RO is installed. Then, downstream of the RO, all returns from the users are collected. The system recircs back through a heater and an expansion tank to the pump suction.

Installation of a control valve instead of the RO is another way to achieve the same result.

Hope that this helps........

MJC

 

[dcasto]

An RO is just a fixed open control valve. The variables for flow through an RO are inlet and out pressure. If the inlet and outlet pressures are constant, then a given RO will pass the same amount of material. A pump has a constant differntil pressure at some flowrate, and RO can be sized to make a pump run at that point ie if it takes 5 gpm on the pump curve to produce an out pressure of 20 psi with a 0 psi inlet, then all the variables are known to calculate the RO diameter.

Try here

http://www.foxvalve.com/frameset-venturi.html

By the way, the equation is also known as the criticle flow equation.

 

[rkc]

You can find some interesting info & equations at

http://www.cuservices.net.

 

[44715]

I am looking for a solution to use the restriction orifice to reduce the pressure of water from water supply wellfrom 117 bar to 8 bar through multiple restriction orifice bank.

suggest me the equations to use for calcualting the orifice size for each stage for a given pressure drop and also without causing cavitation of the water.

 

[poetix99]

44715,

"dcasto" made a good point in saying that "a Restriction Orifice is simply a control valve in a FIXED (opened) position".

This also implies that any operational flexibility is lost for the sake of minimizing the first cost of your "letdown station". It is a safe bet that at some point in the future use of your system, you will want some control - control that valves would have given you, but which fixed orifices will not.

In any case, if your HP water (117 bar)is at a temperature corresponding to an enthalpy that is greater than saturated liquid enthalpy at 8bar, it will flash (cavitate). Staging the throttling process might be desirable for other reasons, but unless you provide additional cooling as you throttle it will not prevent flashing.

 

[TD2K]

44715, I have an article that discusses what you are looking for, sizing multiple ROs in series to avoid cavitation.

If you post an email address, or drop me an email at testdog2000@yahoo.com, I'll email you a copy of the article.

 

[CHD01]

1) Restriction Orifices do NOT control pressure, this is because they cannot control the upstream or downstream pressure. If a valve is closed downstream of the orifice, the system will see the maximum upstream pressure.

2) Restriction orifices also do NOT control flow since flow will vary with upstream and downstream pressure.

3) What restriction orifices do is to LIMIT the maximum possible flow through that orifice (and downstream equipment) OR LIMIT the minimum pressure seen by a pump for a given flow. It does this by the designer varing the diameter of the orifice in the metal plate installed in a pipeline. The size of the orifice is determined by the range of operating conditions that can occur across the orifice. Normally you would size the orifice to LIMIT flow based on the maximum possible range in dP or to LIMIT dP based on maximum range in flow .
4) In the example that MJCRONIN addresses, the orifice is actually being used to increase the TDH required for the system so that the system curve intersects the pump curve for the chosen impeller size. Since the TDH developed by the pump is constant, its sometimes easy to say (for this case) that the RO is controlling flow or pressure - But generically - this is an incorrect statement! RO's CAN NOT BY THEMSELVES CONTROL PRESSURE OR FLOW! The more you learn, the less you are certain of.

 

[CHD01]

I agree with Poetix99, multiple orifices in series to limit dp across orifices will not eliminate flashing.

This can get interesting, since the multiple orifices can sometimes hurt you versus taking all the drop at an exit expansion. This is because of the additional pipe line dp due to 2-phase flow.

DCasto: I think you meant that a pump produces a constant differential HEAD, not a constant differential PRESSURE. The more you learn, the less you are certain of.

 

[TD2K]

Obviously, if the conditions of the inlet liquid are such that it will be 2 phase at the downstream pressure conditions, multiple orifices will not stop the flashing. I assumed the OP didn't imply this.

However, if you are trying to prevent cavitation of the liquid due to the reduced pressure drop at the vena contracta as it passes through the orifice (and the vapor subsequently collapses as pressure is recovered to the bulk downstream pressure), then multiple orifices can help. This is the same principle used in cavitrol trim and other similar valve trims to control cavitation. Basically, the orifices are sized such that the minimum pressure remains above the fluid's vapor pressure.

How practical it is depends to a large amount on the vapor pressure of the inlet fluid compared to the downstream pressure. The closer you are, the more orifices you require.

 

[CHD01]

Agreed TD2K. I'm usually more interested in the total change in fluid properties after an expansion to a collection tank or to atmosphere as opposed to conditions for a specific orifice within a pipe line. But I am curious in how you would determine the spacing between orifice plates installed in series. Is it a rule of thumb like it is for recommended straight pipe length before and after a control valve? Can you please comment on that TD2K. Thanks. By the way, I've enjoyed your comments over the various treads in this netweork of struggling engineers.
The more you learn, the less you are certain of.

 

[TD2K]

CHD01, the article was based on theoretical work done by Tullis and Govindarajan. The spacing between the orifices is a linear relation based on the beta ratio and is the point where full pressure recovery occurs, said to be where the jet from the orifice expands to the pipe's ID.

The article was 'likely' out of Chemical Engineering magazine, perhaps back in the mid 80s, unfortunately, that information didn't make it onto the copy I have. It was written by a pair of engineers working for Ontario Hydro.