Orifice Flow Conditions

[MatthewD]

I am curently designing a orifice plate that is attached to the discharge of a pressurized vessel. Since this is a transient system I am not sure at what point during the depressurization I should use for the designing of the orifice. Any advice would be appreciated.

 

[katmar]

What is the orifice trying to achieve?

 

[MatthewD]

A reduction from 18 MPa to 2 MPa

 

[davefitz]

I think you need to know 4 out of 5 variables to solve for the unknown item.

flowrate, inlet press, inlet temp, fluid composition ( Cp/Cv and spec vol), orifice area.

one out five won't cut it.

 

[katmar]

You need a valve with a pressure control loop - I cannnot see an orifice doing this duty under varying load.

 

[MatthewD]

Thats what I thought. I just think tht a 16 MPa differntial is too much for a orifice

 

[sailoday28]

What is the purpose of the orifice during the transient?

 

[MatthewD]

Well normally you would just vent off the gas and drain the liquids to a tank. However in this situation people are considering operator error in which case the valve to the drain tank would be opend with out venting off the gas first. So they want a orifice plate to try and take some of the pressure off before it hits the tank.

 

[mbeychok]

MatthewD:

So you are talking about installing an orifice in the liquid drain line just upstream of the liquid drain valve? Is that correct?

Milton Beychok
(Contact me at www.air-dispersion.com)

 

[MatthewD]

Yes that is correct

 

[mbeychok]

MatthewD:

One more question. What is the liquid? Will it partially or completely flash into vapor when the pressure on it is reduced from 18 MPa to 2MPa (i.e., 178 atmospheres to 20 atmospheres)?

Milton Beychok
(Contact me at www.air-dispersion.com)

 

[khan101]

Restriction orifice(RO) is the crudest form of regulators or control valves. It act as regulator only if flow is maintained. In staic condition RO does not have any effect. ROs give considerable resistance to flow. Its down stream pressure depends on upstream pressure. RO also can be called as fixed choke. Fixed chokes are very common in lube oil lines of engine etc. It is the most cost effective method if upstream pressure is more or less constant.

Improved methods to control flow are variable choke, HCV, self regulated CV/regulator and control valve respectively in terms of improvement.

If you are building a RO, with drop comparable to the pipe pressure rating, then the RO should be about as thick as a blind flange of the pipe rating. That saves picking the pieces out of the downstream equipment. And a RO does
NOT act as a pressure reducing valve. Stop the flow and downstream pressure equals upstream pressure.

 

[MatthewD]

Since it is natural gas it will only flash partially about 1 % by my calcs.

I agree with you khan as well if it was up to me I would use something a little more sophisticated but cost factors are importent.

I have decided to take the conditions immediatly after the valve is cracked as it would be the most stressfull on the orifice ( Lowest T and Highest delta P) and thus make the best design case. As to whether or not any RO can provide the pressure drop I require I am still not sure.

 

[mbeychok]

Matthew:

You haven't told us how often the upstream vessel is depressurized and drained. Assuming that it doesn't happen too often, you might consider this:

(a) Chain lock the liquid drain valve in the closed position and fasten a sign to the piping adjacent to the valve that says "You must not unlock this valve unless the upstream vessel has first been vented and depressured. When that has been done, unlock and open this valve very slowly.".

(b) The operator must obtain the key to the lock from his/her supervisor.

Are you quite sure the liquid will not flash more extensively? It seem to me that subjecting raw natural gas to a pressure of 180 atmospheres may cause some propane, butanes and heavier to condense (unless the gas is quite hot). If the liquid does contain considerable propane and butanes, it will flash when the pressure is reduced and the resulting reduction in temperature may cause the drain valve to freeze. If the natural gas has been processed to remove propane, butanes and other natural gas liquids ... then disregard this comment.

Milton Beychok
(Contact me at www.air-dispersion.com)

 

[sailoday28]

As a first approximation consider no heat transfer to or from the tank.
Second approximation, the pressure as a function of time is equally distributed thoughout the tank.
Third, neglect KE of the components in the tank
The change in internal energy of the liquid and vapors in the tank
DELTA{(Mf*Ef)+(Mv*Ev)}=integral of Wf*Hf*dt
where Wf is the liqid flow rate
Hf is the specific enthalpy of the liquid in the tank
t is the time.
On a simple numerical integration,given a pressure in the tank, the orifce flow should be known. Use that flow over a small increment of time.
This allows calc of change of internal energy in tank.
With the change of internal energy of that time, calculate a new pressure (if two phase, one component-the problem is greatly simplified)
The new pressure and temp allows calc of new orifice flow.
repeat process.
One must check to see if orifice flow is restriced by flashing, choking, etc.
With P vs time known, repeat calc with a smaller time increment, until P vs time is reasonably repeative.
I'd hate to do this simplified problem by hand.

 

[MatthewD]

So my understanding of the problem has changed a bit. After a pig arrives the drain is opend for a few seconds under full pressure to push out all the liquids into the tank. The restriction orifice is to make sure that the increase in pressure of the drain tank does not excede 1 atm.

The volume of the Reciver is about 4.8 m3
The drain line is 2 in.
The Tank is design for atmospheric pressure with a 4" vent to atmosphere
The Natural gas is mostly dry methane at 14 mpa

I need to figure out what the delta P across the RO should be to keep from blowing up the tank. No one at work has been able to help me with the calcs needed. I have to do all the orifice sizings by hand. Any help would be appreciated

 

[sailoday28]

MatthewD (Chemical)states:
The Tank is design for atmospheric pressure with a 4" vent to atmosphere
The restriction orifice is to make sure that the increase in pressure of the drain tank does not excede 1 atm.
With any inflow to this receiver tank, there will be some outflow via the 4" vent. To drive flow via the vent, the receiver tank pressure (neglecting gravity)will be greater than 1 atm.
Perhaps you can pick various orifice sizes, assumed choked flow for the few seconds.
This allows calc of mass and energy entering the receiver.
Make a simple thermal hydraulic model of the receiver.
Flow energy in -flow energy out(via the vent)=change in tank internal energy. Neglecting heat transfer
This will give a handle on vent flow and dp across vent line and therefore pressure rise above 1 atm in the receiver.
sailoday_28@yahoo.com

 

[sailoday28]

Please modify my energy balance statement for the receiver The left hand side should be multipled by the time increment of "a few seconds".

 

[mbeychok]

MatthewD has not yet given us a really complete description of his system and, without that description (or a flow sheet), it is difficult to give any meaningful advice. From what he has said, it seems that he has a gas-liquid separator tank that receives the flow of gas, liquid and sediment pushed into the separator tank by pigging a gas pipeline to clean the line. The liquid from that separator is then manually drained into a vented atmospheric pressure drain tank.

If the above description of his system is correct, then it should be said that, in many pipelines, such pigging operations occur quite sporadically ranging from perhaps once a month to once a year. If that is the case for MatthewD's system, then perhaps instead of attempting a physical fix such as a restriction orifice, he should consider an administrative fix such as I suggested on November 17th:

(a) Chain lock the liquid drain valve in the closed position and fasten a sign to the piping adjacent to the valve that says "You must not unlock this valve unless the upstream vessel has first been vented and depressured. When that has been done, unlock and open this valve very slowly.".

(b) The operator must obtain the key to the lock from his/her supervisor.




Milton Beychok
(Contact me at www.air-dispersion.com)

 

[jay165]

MattD:

What you want is a "choke nipple". It is a small length of pipe with a 1/4" bore. It is very common to install these devices on a drain from a high pressure source to a low pressure source. Sorry, but I couldn't find any website for sales/reference.