11 replies to this topic

Hi

How do you thnk shut off head affects the pump discharge head.
Lets say I have a pump with inlet head of 2 m and a differential head of 3 m.My shut off head is 4 m. What discharge head will I get? 5 m or 4 m

Thanks in advance
Gyan

[breizh]

Gyan ,

H = f (Qv) .
Shut off means no flow !

Hs = 2m ; Hd-Hs = 3 m>>> Hd = 5m

Breizh

Thanks but can the discharge pressure be more than the shut off pressure? In the same case which I have mentioned above how high the suction head can go and still delive the the discharge head, which will be suction head + differential head?

Edited by gkumar, 08 November 2012 - 01:40 AM.

[breizh]

Take a look at your pump curve , you can read the shut off head on the left side of the curve .
The operating point is at the intersection of the pump curve and the system curve .

Breizh

Thanks Breizh

Lets sat I have inlet suction head of 1.5 m and a differential head of 2 m. I will get an out put of (2+1.5) 3.5m. Now if the pump's shut off head is 3.8m and I increase the suction head to 2.5 m, will I get a discharge output of 4.5 m or it will not go above the shut off head?

[fallah]

gkumar,

Shut off head is differential head at pump shut off and with a suction head of 2.5 m and shut off head of 2 m you will get 4.5 m head as pump discharge head at shut off condition.

Fallah

[breizh]

Consider this resource to support your query .

Breizh

[breizh]

Consider this resource to support your query .

Breizh

gkumar,

Shut off head is differential head at pump shut off and with a suction head of 2.5 m and shut off head of 2 m you will get 4.5 m head as pump discharge head at shut off condition.

Fallah

Hi Fallah

Thanks for the reply!

I believe in place of "shut off head" (colored portion) you meant "differential head". Isn't it? Can my suction head go above my shut off head? Yes or No.

[Dacs]

You're confusing terminologies that I get confused myself

First of all, you have a pump with a known performance (aka characteristic curve). From this, you can get the differential head for a given flowrate.

Shutoff head, as implied by the word itself, means head at no flow.

For a given impeller and a given pump (and fixed RPM), this head is unique.

You have to realize that the pump curve is flow vs differential head (discharge - suction head). You can't get the discharge head by looking at the curve alone. You also need to know the suction head and add the corresponding differential head to obtain the discharge head.

So basically, suction head and differential head are not comparable with its other (former is a function of system configuration such as elevation, pipe length etc while the latter is a function of the pump performance itself). So yes, suction head can be higher than shutoff head (as defined in this post as the differential head at no flow) by making the source vessel high enough, but it won't make any sense to compare the two because one is an absolute term and the other one is a difference term.

But if by "shutoff head" you mean the actual head produced by the pump at zero flow (which means suction head + differential head at zero flow), then no, it's not possible that shutoff head < suction head.

Thanks Breizh

Lets sat I have inlet suction head of 1.5 m and a differential head of 2 m. I will get an out put of (2+1.5) 3.5m.

In this case you have suction head of 1.5m and a differential head (at the same flow) at 2m, so your discharge head is 3.5m (as you correctly state).

Now if the pump's shut off head is 3.8m and I increase the suction head to 2.5 m, will I get a discharge output of 4.5 m or it will not go above the shut off head?

This is where all the confusion arise.

By fixing the 3.8m shutoff head even though you have an increase in suction head implies that this value is in fact the differential shutoff head.

In this case, if you increase the suction head to 2.5m (by elevating the vessel or increasing the source vessel pressure), the corresponding discharge head would be 2.5m + 2m = 4.5m.

Looks like we exceeded our shutoff head!

NOT!

Because (as I already said) the shutoff head (3.8m) is differential head, so to get the absolute shutoff head, you have to add 3.8m to the suction head at NO flow (which is NOT equal to 2.5m since this corresponds to a particular flow which is not zero).

At any rate, you just increased your shutoff (absolute) head by increasing the suction head, but the shutoff (differential) head will remain the same unless you increase the pump RPM, impeller diameter or replacing the pump altogether.

TL;DR version: No, discharge (or suction for that matter) head will NEVER exceed shutoff (absolute) head.

Edited by Dacs, 09 November 2012 - 12:43 AM.

[fallah]

I believe in place of "shut off head" (colored portion) you meant "differential head". Isn't it? Can my suction head go above my shut off head? Yes or No.

gkumar,

Yes, the suction head can go above pump's shut off head.

Fallah

[kkala]

In addition to above, it is pointed out that shutoff pressure at pump discharge is always greater than suction pressure. Pump shutoff pressure in case that suction pressure is maximum (e.g. HHLL of a suction atmospheric tank) is the maximum shutoff pressure, which is often considered as design pressure of discharge piping.
Clarifications on pump "heads" can be seen in Perry, Transport and storage of liquids / Pumping of liquids and gases / Introduction (page 10-22 in 7th edition).
Suppose in your example (post 5) that shut off pressure =3.8 m L g, when suction pressure is 1.5 m L g and discharge pressure is 1.5+2.0=3.5 m L g at normal flow.
When suction pressure =2.5 m L g , shutoff pressure will be 4.8 m L g, since shutoff (differential) head remains constant at 2.3 m L
(in above 2.5 m L g means 2.5 m of liquid column above atmospheric pressure; 2.0 m L may be too small for pump diff pressure, but does not mind the example).
In pump data sheets here, we specify suction, discharge, shutoff pressures in gauge pressure units (e.g. Barg) for clarity; "head" is only used for pump head (expressed in m of liquid), corresponding to pump differential pressure.

Edited by kkala, 09 November 2012 - 01:42 PM.