Pipe calculation - pressure, flow and orifice size

[MarSmall]

Hi everyone,

I am looking at a system and I need to calculate what the correct orifice size should be for two orifices. I have a pump (Point 1) discharging 117m3/hr at 2.7bar.

The orifice at Point 2 needs to be sized so that branch supplies 8.5m3/hr to its consumers.

The orifice at Point 3 needs to be sized so that branch supplies 105m3/hr to its consumers.

Using conservation of mass I have assumed that Point 4 supplies 3.5m3/hr

In order to calculate the size of the orifice, I believe I need to calculate the pressure drop across the orifice. If I assume the pressure on inlet to the orifice is equal to the pump discharge pressure/system (2.7bar), then all I need to do is calculate the pressure on the discharge side of the orifice – I am stumped on how to do this. I have tried using Bernouli’s equation, but I end up with 2 pressure unknowns (The pressure on the discharge side of the two orifices) and cannot solve. What am I doing wrong? Or is this unsolvable?

 

[SandCounter]

Calculate the loss in the 4" line for 105 m³/hr up to the branch without the orifice. Calculate the loss in the 1-1/2" line for 3.5 m³/hr up to the branch. Note the difference in head loss. This difference is what you will have to take up with the orifice. Use the difference in head loss to back calculate orifice size from known (target) out flow.

Do it again with the first 1-1/2" branch. Calculate the loss in the 1-1/2" line for 8.5 m³/hr and add the loss from the higher of the two branches calculated above plus the loss in the distance between the branches for 108.5 m³/hr. Note the difference in head loss which indicates what you will have to take up with the orifice. Use the difference in head loss to back-calculate the orifice size from known target flow.

The key here is that there is a single pressure at each node.

I used to count sand. Now I don't count at all.

 

[bimr]

One would expect that it will not be practical to obtain the precise flow splits that you are describing with just the use of orifice plates.

 

[MJCronin]

Why would you NOT use a combination of control valves or manual valves ? Orifice plates are not precise and require expensive flanges

Control valves permit adjustibility should process conditions change ....

MJCronin
Sr. Process Engineer

 

[LittleInch]

A few things,

Your pump flow if a centrifugal pump, will vary flow with pressure so the 117 at 2.7 is just one of many possible flows and pressures.

But I think you're right - unless your orifice plates and pressure drops become critical flow where downstream pressure becomes irrelevant if maintained below a certain pressure.

As others say , orifice plates to get something as accurate as you're trying to do is not practical.

E.g. How will point 4 not just flow maximum flow? What is stopping the flow all going that way?

Orifices need to be art of a system and you only have half the system here. You need it all the way to end user.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[dean427]

Hi

It is not impossible - you have the information that you require.

You have defined the required flow rate and the upstream pressure and therefore you can either assume a downstream pressure to calculate the orifice size for the flow you require to pass or else assume an orifice size to calculate the downstream pressure required to give you the flow through the orifice.

You should refer to Crane TP410 for the basic liquid phase orifice sizing equations. Or the more accurate method these days is to use the HDI method if you have access to a process simulator it is quite easy - your process engineer should be able to help with this.