Max Pressure with two pumps working 1

[nabeel3]

I have the following situation. I want to determine hydro test pressure for the piping system, which shall be 1.5 times the max pressure(possible) in the system. There are three vertical turbine pumps, in which max two may be working for washing sea water drum screens. TDH for the pump is 52 m and the static discharge head is 7.8 m. so I am considering (52-7.8=) 4.4 bar as the maximum pressure at the discharge flange. But we have the case of two pumps working in parallel also. See the pump curve. In this case, what is the maximum pressure developed in the system for me to calculate test pressure.

 

[LittleInch]

nabeel,

You have fundamentally misunderstood the issue I'm afraid.

Normally the maximum possible pressure is a dead head (closed valve or blockage downstream the pumps).

So this is normally taken as the differential head at no flow - in your case 79m?

Also is your static discharge head fixed? Or does it change with the tide?
And this is normally measured from the pump discharge flange so static head shpuldn't come into play, but a diagram would help.

Normally the process engineer works this out and specifies the design pressure or MOP and test pressure is 1.5 times the design pressure, but this does depend on your design code which is ??

Anyway 4.4 bar is far too low.

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

 

[nabeel3]

Dear Little inch

Thank you for your advice. We have considered low tide for calculating the suction head. please see attached file.
Yes, I understand that I made some mistakes. Below is the email communication between me and the client.

My Question 1

Subject: Technical Query for Hydro test pressure of Wash Water Piping

- In continuation with trailing emails and attachments herewith, the contractor hereby informs Marafiq that the maximum pressure developed in the discharge flange of the vertical turbine pump is 4.4 bar and the available pressure for the washing sprinkler is 3 bar. In this case, the contractor proposes to conduct a hydro test for piping except for the sprinkler at 1.5 times max pressure, i.e. 6.6 bars(1.5X 4.4). Subsequently, piping support calculation shall be based on hydro test pressure

Client Answer 1

The source for the mentioned 4.4 bar is not clear.

As per the pump data-sheet and pump curve, the pump shut-off pressure is 7.9 bar (Say 8 bar). This should be the minimum design pressure of the wash-water piping. Accordingly, the hydro static test pressure of the wash-water piping should be 8 x 1.5 = 12 bar

My Reply to above Answer

As you mentioned 8 bar is the shut-off pressure. Since shut-off pressure is the pressure developed at zero discharge, so there is no point in considering shut-off pressure for the pipe hydro test.

We have to consider the maximum pressure available at the pipe inlet flange( pump discharge flange), since in our situation the pump discharge flange is at a higher elevation, which translates into the static head of 7.8 meters, please see the attached drawing. Hence I am considering the maximum pressure available as 52m-7.8m head, which gives 4.4 bar for any pump at discharge flange. That will be the max pressure for the Piping circuit.

 

[nabeel3]

Please see the attached piping arrangement. The pump impeller is at an elevation below the discharge flange sucking water from a chamber to clean the Drum screens.

 

[LittleInch]

Your client is correct.

You need to use the worst case conditions.

This would be high tide and no flow.

That's how it's done.

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

 

[LittleInch]

"so there is no point in considering shut-off pressure for the pipe hydro test."

This is the key point where you are not correct in nearly all situations.

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

 

[nuuvox000]

You have to plan for a dead-head situation. Someone could close a valve and then your pipe would leak or break if you don't plan for it.