operating pressure 1

[kdashc]

I have graph for a pump that shows the flow rate versus a lot of different values. one of those values is the total head (labeled T.D.H.). So if I assume that i have flow rate of 30 gpm this coincides with a TDH of 130ft.

Now to figure the operating pressure of my liquid at the pump discharg is it correct to take that 130ft and multiply it by the density and gravity to figure this operating pressure?

ie P = (TDH)*g*p for instance if i have water (p=62.4lbm/ft3) and use g=32.2ft/s2 then i'll have an operating pressure of 56.37 PSIG?

thanks for any help

 

[25362]

TDH is total differential head between discharge and suction. Thus the suction head (pressure) is a factor in determining the discharge head (pressure).

 

[BigInch]

As 25362 says, add the suction pressure.

You should also look at what the maximum pressure may be for the discharge line in question. That would be the suction pressure + pump shutoff pressure (the equivalent pressure of the pump's differential head at zero flow). It may be possible to reach that pressure in the event that a valve is closed (intentionally or mistakenly) or if a plug is formed somewhere in the discharge piping. If the pipe is designed for less than the maximum shutoff pressure, it would be prudent (or required by code) to consider installation of control and/or relief valves to make safe operation possible.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[ione]

TDH is the total dynamic head and to quote the Pump Handbook (J. Jarassik) it is “the increase in height of a column of liquid that the pump would create if the static pressure head p/rhog and the velocity head V2/2g were converted without loss into elevation head Ze at their respective locations at the inlet to and outlet from the control volume; that is, both upstream and downstream of the pump”. Take care when you have to deal with fluids other than water and play with consistent unit of measure.

 

[ione]

An example could enlighten

 

[BigInch]

ione,

I notice that the example discusses accelaration velocity head in the suction line, but does not actually include it in the calculations.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[ione]

BigInch,

I checked again the example posted earlier. This is not a trivial omission: velocity head should be taken into account (they shouldn’t practice what they preach)

 

[BigInch]

Do you like this one?

http://www.lightmypump.com/help.html

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[kdashc]

ione,

So in your example the total dynamic head is calculated and can that be then used in pump diagram that gives tdh versus flow rate.

Then for example if this particular diagram says that for a tdh of 100ft, from the example, there is a flow rate of 30gpm.

Then the pump will be producing a flow of 30gpm and from that you can calculate the dynamic pressure = 0.5 x p x v^2, and is this what they call operating pressure.

Is my reasoning correct?

 

[ione]

The process should be reiterative, as the calculation of the TDH asks for the flow rate (to determine friction head and velocity head).

 

[kdashc]

ok ione, that makes sence. But what if I have a certain configuration (ie TDH of 100ft) and a certain pump (ie 30gpm at 100ft) and i want my flow to decrease to 20gpm.

how would i do that?

vfd?

 

[ione]

Pumps delinked from the systems they operate are somewhat meaningless.
The performance curve of a pump (supplied by the manufacturer) represents the set of two parameters which are linked one to other (flow rate and head). But the pump curve by itself is useless. When the pump is fitted in a system, the pump’s operating point is determined by the peculiarity of the system (pipeline, fittings etc). Each system produce a particular response to a given fluid flow rate. The operating point is determined by the intersection of the pump’s curve and the system’s curve.
Now if your requirements are changed (reduced flow rate), you have two opportunities available.
1) Act on the system. Throttle the discharge line (do not work on suction line as this could lead to cavitation issue), thus modifying the response of the system for a given flow rate.
2) Act on the pump. Change the speed (VSD, VFD) thus reducing energy input.

Take a look at the attached file and enjoy it!

 

[kdashc]

thanks ione, one last time tho just so I know whats going on. The term that i hear all the time of the 'operating pressure' of the system at the discharge side of the pump can be calculated by pv^2/2 (which is also called the dynamic head?)

 

[kdashc]

...or does it consider static pressure? I guesse now i can just pose my question as to how is the 'operating pressure' calculated?

 

[BigInch]

Shutoff, Normal, Operating, Maximum Pressures, or any gage pressure is calculated by fluid density times the head at the given condition, for example at a pump shutoff head of 100 feet and water as the fluid gives a pressure is 62.4 * 100 or 6240 lbs/ft2 = 43.3 psig. For another fluid, simply multiply that result by the specific gravity of the other fluid. Gage pressures do not measure and hence do not include the velocity term. The hydraulic grade line is the sum of all heads at any point along the system and do include both pressure head plus velocity head.

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BTW, in addition to adjusting speed and throttling, you might divert some flow back to suction via a recirculation line thereby continuing to operate the pump at BEP, but allowing only some flow to the discharge piping and diverting any excess back to suction.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[ione]

To be rigorous velocity head should always be taken into account. In some cases it gives such a small contribution it can consequently be disregarded, but it is there and plays its (even small) role.