Restriction Orifice for Water Pipe 1

[yfariadi]

Hi,

I have an oversized centrifugal pump that I need to bring back to its duty point. Main header line diameter for both suction and discharge is 12" pipe.
Pump Design Duty Point: 750m3/h, 40m head
Site Reading: 960m3/h, 32m head [pressure head is taken from differential between discharge (35m) and suction (3m) pressure gauge of the pump. Flow rate is from the pump curve at 32m head]

In order to increase the head from 32m to 40m, I installed orifice plate with 8" opening diameter. I calculated the required orifice plate diameter based on differential pressure of 8m, flow rate of 750m3/h, and orifice K-value of 0.6. The formula is based on the website below
[URL unfurl="true"]http://www.engineeringtoolbox.com/orifice-nozzle-venturi-d_590.html[/url]

After running the pump again with orifice plate this time, the discharge pressure is only increased to 38m while suction remain the same at 3m. I still cannot increase the pump pressure head to 40m. Can please advise what is the correct orifice size required?

Regards,
Yulius

 

[bimr]

Here is another orifice calculator.

http://www.tlv.com/global/TI/calculator/water-flow-rate-through-orifice.html

It looks like a 7-Inch orifice plate would work.

 

[yfariadi]

Hi bimr,

Thank you for your reply. Is there any way to calculate the pressure increase when the system without the orifice and with orifice? For example, without orifice is 32m head and what is the orifice size required if we want to increase the system to 40m head. Most of the calculations provided in the internet is just to measure pressure drop across orifice.

Regards,
Yulius

 

[bimr]

The pressure without the orifice may be calculated as it is the sum of the friction losses of the various components in the piping system.

 

[katmar]

Yulius, you are on the correct track when you want to be able to calculate the pressure drop at the different flows with and without the orifice plate. The critical aspect of this calculation is to distinguish between the head caused by friction and the head caused by static height difference because they respond in different ways to changes in flow rate. The friction head varies with approximately the square of the flow rate, but the static head does not change with different flow rates.

For the sake of example let us assume that when you have a flow rate of 960 m3/h the 32 m of head is made up of 10 m of static head and 22 m of friction through pipe, valves and fittings. When you drop the flow rate to 750 m3/h the static head will still be 10 m but the friction head will decrease to 22 x (750/960)^2 = 13.4 m. Your total head at a flow of 750 m3/h would therefore be 13.4 + 10.0 = 23.4 m. To balance with your pump duty point of 40.0 m you therefore need to add in 16.6 m when the flow rate 750 m3/h.

A restriction orifice needs to be 5.68" diameter to generate a permanent pressure loss of 16.6 m at a flow rate of 750 m3/h according to Hooper's formula as implemented in AioFlo.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"