Pump upstream and downstream design pressure ?

[silebi66]

Dear Ladys and Gentlemen at Eng-tips :

I have confused for a long time to know why the design pressure of the pump upstream and downstream to be equal to the pump shut off pressure.
(attached drawing is typical example I found on the Google)
The max. pump discharge press = max. suction press + 1.2 * pump shutoff diff press = Design pressure in
this design guide. But, it is impossible that the max. pump pressure = Design pressure.

In my understanding, process engineer can first estimate the pump shut off pressure
by using the rule-- 125% of the differential pressure at normal capacity. And, this shut off pressure can be used
as design pressure at the pump upstream and downstream before the valve.

Is any fault in this logic ?
Please kindly help me out or shade some light on this issue.

Greatly appreciate your suggestion.
Thank you.

 

[Drexl]

Hi,
It is a bit hard to follow your text. There is no absolute rule for determining design pressure of these lines.

The reason why the shut-off pressure is used generally for determining the design pressure is of course that if you have a control valve which may close you approach the suction pressure + shut-off pressure. Generally the design pressure at suction side is much lower, and in case there is a risk of back-flow, it is to be protected by a safety valve or similar measure.

If you are implying that design pressure should be selected as 1,25 x HEAD, this is wrong. You have to take suction pressure into account.

 

[LittleInch]

Design pressure needs to be the highest pressure that a piece of pipe / quipment can see during normal operation, including shut off conditions. For pumped systems this commonly depends on the pump characteristics and inlet pressure. Before the pump is purchased, a value of 1.2 x operating differential head is a good start point. To this you need to add the max inlet head which could occur. However design pressure may be affected by other things such as surge pressures, thermal loading, other connections / future use / relief valve settings, pressure trips etc so whilst the pump head is the initial start point, it may not in the end be how the design pressure is established.

At what point you change the design pressure accross a pump is dependant on your system, but commonly if you have an inlet isolation valve, the rating of this is the same as the discharge side because if the outlet valve is opened before the inlet valve and pump is in operation, then the inlet side can see the outlet pressure even with a non return valve.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[silebi66]

Thank you Drexl and LittlInch Sir's comment.

I would like to post my thought on this, but not sure it is logical.Please kindly give me any suggestion if you are available. I will appreciate your help.

[Case 1].
If the pump suction side is still in the running, the operator accidentally close the downstream valve. The pressure at pump discharge side will buildup to the pump shut-off pressure.

[Case 2].
If the pump suction pressure build up to the design pressure(ex. the operator accidentally close the pump upstream valve?), the pump is still
running, and the pump downstream valve is open,

[Case 3].
If the pump suction pressure build up to the design pressure, and the operator also accidentally close the downstream valve. But, in this case, the surge pressures may be estimated not exceed 10% of the design pressure.
As long as the relief valve is installed on the pipe line or pump. the set pressure of the relief valve to be set to equal the pump shut-off pressure to protect the
the pump.


The design pressure = 1.2~1.3* delta_P + P_suct.---------[Case1]

The design pressure = delta_P + H + P_disch.--------------[Case2]

The design pressure =1.2~1.3*delta_P + H + P_disch.---------[Case3]

Where
delta_P = pump discharge pressure - pump suction pressure.

P_suct. = normal operating pressure at pump suction.

H= the liquid supply level above the pump center line(static height).

P_disch. = pump discharge pressure.


The real shut off head of pump must be refer to the pump performance curve from the pump vendor. The designer or engineer can find the max differential head and pressure at zero flow from pump head-capacity curve, and add the max pump suction pressure to get the real pump shut-off head.

Thank you.

 

[LittleInch]

You're making this too complex. Case 1 is Ok, but 2 and three adds the pump pressure twice (already in the delta P and then also the P discharge) This is not physically possible.

Setting design pressure is a factor of many things - one of which is the pressure seen at "normal" flow. How you cope with excursions from max operating is own to many factors inlcuding relief philopsophy, whether other quipment in the same system can accept higher pressures or the quality of your shutdown system. IMO, a system should be able to cope with pump shut off head plus highest inlet pressure, at least until the next control valve, but there are differeing views on this.

Piping tends to go in pressure jumps based on the flange rating so this can have an impact if you're close to the boundary points, especially #600/ 900 and 900/1500.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[MortenA]

@Littleinch
Its still common to have the design pressure for the pump casing selected at a specific value while the piping may follow the pipespec/flange rating.

@silebi66: Its unlikely that a PSV/relief valve will save you from a surge (water hammer). They are too slow and dont have the right performance. Special anti-surge valve (Neles Jamesborough and Mockweld makes them, maybe also others) may do the trick. But this will require special analysis if its a real threath.

Best regards, Morten

 

[130684]

Hola,

design pressure (DP) of discharge line is usually = the set pressure of the upstream relief valve (SP) + pressure given by the static height between maximum liquid level (HLL) in the upstream vessel and pump centerline + shut-off pressure

a conservative approach to estimate the shut-off pressure is to consider that the shut-off head is 1.3 heigher than the rated point (or 1.2 x normal operating point).

therefore, DP(bar) = SP + HLL*rho*g/1e5 + 1.2*DH_normal*rho*g/1e5

The suction design pressure is sometime considered equal to the discharge design pressure because while one pump is running, the spare one can have the discharge isolation valve open (operator error)...but is considered equal, only up to the suction isolation valve (the section between the suction isolation valve and pump).

Regards,