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Multi hole orifice in Sea Water return Line

Multi hole orifice in Sea Water return Line

(OP)
In Our site there are total seven plants and Sea water is supplied to some of the plants. The first plant near to the sea water supply having issues in return line. Because of high pressure drop available, we have inserted a multi hole orifice in the return line (72"). Then we found flashing of water at the exit of return line like a heavy jet. Then we have inserted one more multi hole orifice at the end and distributed the available pressure drop (~3.5 Kg/cm²).

Now the problem of choking is happening frequently. Pipe is FRP pipe and we found in the last shutdown the pipe inner wall liner got removed and strucked in holes of orifice along with sea shells. We have made a temporary bypass (24" line with one more multi hole orifice) across the second orifice, through which we have got some improved flow.

Data
required flow is 14500 m³/h
Existing Pipe Line is 72"
Two multi hole orifices
First orifice pressure drop 1.5 Kg/cm²
Second Orifice Pressure drop is 1.7 Kg/cm²

My question is why a multi hole orifice is required? Why vibrations in Pipeline happens, when we insert a single hole orifice. Can we insert a single multi hole orifice instead of two?

Is there any special sizing calculations required for Sea Water Lines. I have read that for gravity lines, we may need to follow Froude number < 0.3. Her Sea Water supply is coming from a pumping station and then it passes through series of filters and exchangers and finally returned to sea through return line.

Thanks
Rajesh

RE: Multi hole orifice in Sea Water return Line

That's a big flow and a big pipe. At that pressure, to get the pressure drop you need with a single orifice, your flow will be critical, or nearly critical and hence be at or close to sonic velocity. Your flow will almost certainly be cavitating and creating lots of bubbles and turbulence, hence the shaking of the pipe. Multi hole plates distributes the flow disturbance over many holes.

What you really need is probably a multi plate restriction orifice or a control valve.

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

RE: Multi hole orifice in Sea Water return Line

The multi-hole plates have cavitation coefficients that are 3 to 4 times lower than the cavitation coefficients of the single hole plates. This is the reason why you have high vibrations and noise if you install single hole plates instead multi-hole plates.
As say your post, the two orifice plates must produce a total pressure drop of 3.2 kg/cm2. To achieve an optimal design take into account the following criteria:
1) In the direction of the flow, the first multi-hole plate must have more pressure drop than the second, so their holes must be of less size. Design for example the first multi-hole plate for the 60% of the total pressure drop and the second for the 40%.
If now the first orifice has 1.5 kg/cm2 and the second 1.7 kg/cm2, change the position of the plates in order to have 1.7 kg/cm2 with the first plate and 1.5 with the second.
2) If with this change the problem still persists, it should be necessary to design again the orifices to have the first approximately 1.9 kg/cm2 pressure drop and the second 1.3 kg/cm2.
Likewise if it is possible, install the orifices at the beggining of the discharge piping in order to have the maximun length of the downstream piping to create more backpressure in the second orifice.

In the past in the essential service water system of a nuclear power plant with 36" lines I installed 2 multi-hole orifice plates that provoke a pressure drop of 3 kg/cm2. As we were still in the limit of the incipient cavitation in the second plate, we installed also a pice of piping of alloyed steel of 5 m length, downstream the second orifice in order to avoid the erosion that we had in the original carbon steel piping.

RE: Multi hole orifice in Sea Water return Line

@casflow

How did you determine the length of piping to replace with alloyed steel, downstream of the orifice? Was it based on the length of damage observed in the carbon steel?

RE: Multi hole orifice in Sea Water return Line

(OP)
Dear Casflo,

Thanks for your reply.

I am looking for the source material. As you mentioned, first orifice should contribute 60% of the pressure drop and the second for 40%. What happens if it is reversed? Is it mentioned anywhere? Can you please address or provide the source material or can you please send me to my mail address (rajesh.chemiitkgp@gmail.com).

Thanks

RE: Multi hole orifice in Sea Water return Line

(OP)
One more thing is, why vibrations occur, if we install a single multi hole orifice with 3 Kg/cm² pressure drop?

Thanks

RE: Multi hole orifice in Sea Water return Line

To AWloo:The 5 m length is based on the experience. More or less corresponds to 5D because in my case the pipe is of 36".
To patibar: When you install more than one RO (Restriction Orifice) in series in order to divide the total pressure drop, the last RO has less back pressure, so the possibility of cavitation is greater than the others ROs that are installed upstream. Take into account that to know if a RO or a throttled valve has cavitation you must compare its cavitation coefficient Ci with the cavitation parameter S, that is: S = (P1 - Pv)/(P1 - P2)
P1 is the pressure before the RO, Pv is the fluid vapor pressure and P1 - P2 is the pressure drop in the RO.
As the last RO has the lowest value for P1 this is the reason you must design the second RO with less pressure drop.
The noise and vibrations are associated to the cavitation. If you have strong cavitation, you'll have noise vibrations and damage in the downstream pipe if its material is carbon steel.
I will send a paper about multi-hole restriction orifices to your mail.

Best regards
casflo

RE: Multi hole orifice in Sea Water return Line

Your experience conflicts with my "logic" for some reason: I keep arguing to myself that the second of two multiple orifice plates should cause 60% of the pressure drop, and the first cause 40%. That way, you would be less likely to have cavitation behind either the first or second orifice plate.

Is there any advantage gained by inserting bends between the orifice plates to induce additional flow resistance?

RE: Multi hole orifice in Sea Water return Line

To racookpe1978(Nuclear)
To understand why the first plate must have more pressure drop than the second, follow this reasoning:
If the two plates were equal with the same number of holes and with the same hole diameters, they have the same cavitation coefficient Ci. Asuming the fluid is cold water, we can take approximately Pv = 0, so the cavitation parameters of the plates will be S1 = P1/(P1 - P2) for the first plate and S2 = P2/(P2 - P3) for the second plate.
P1 and P2 are the pressures before the first and the second plate and P1 > P2.
The pressure drop in the plates will be the same, that is P1 - P2 = P2 - P3.
In conclusion S1 > S2.
To avoid the plate cavitation must be S > Ci so S2 is more close to Ci than S1. For this reason in the first plate we can reduce S1, increasing P1 - P2 to move away S2 in the second plate from Ci.
The response to your second question about inserting bends between the orifice plates is no. Always is better to have the maximum possible length of pipe after the second plate in order to increase the backpressure P3, because if you increases P3, also increases P2 and of course S2.
Regards

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