Static Discharge Head 1

[Ashjanj]

Hi,

In determing the pump discharge static head discharging into a pipeline leaving the platform, am considering using the length of the vertical lift of line from pump discharge + TOS (Top of steel reference for the platform) to the centreline of the pump. The pump is a new one to replaced the old existing one.

Kindly inform me if my steps can be used. Suggestion is welcome

 

[djack77494]

Ashjanj,
I'm not sure what you're attempting to do here. Your starting point should be the pump's centerline, as you identified. If I understand you correctly, you're proposing the use of the highest elevation on the platform to calculate the static head. Why would you do that? Surely you will not be plumbing the pump's discharge to this high point. Use the actual (or designed) pipe routing for determining the static head you need to overcome.
Doug

 

[Artisi]

The pump static head is the difference in level from inlet water level to the highest point on the discharge side.

 

[Ashjanj]

Doug,

what i did was to use the vertical pipe routing from the pump discharge to determine the static discharge head.

The discharge pipe run vertically upward and it enters an elbow to run horizontally before it then turns downward to enter the pipeline.

I hope my suggestion is right.

Find below my calculation:

The existing vertical pipe discharge run plus the TOS at the pump discharge flange is used to determine the static head for the discharge line.

TOS elevation to the pump discharge = 15.64 m
Vertical run from discharge flange = 6.065 m
Static Discharge Head = 15.64 + 6.065
= 21.705 m (= 71.21 ft)

 

[sshep]

The calculation you are doing represents minimum deadhead for start-up only. A pump transfering between two tanks would at least need to deadhead above top of steel, but only to start-up. Once the line is packed, the operating discharge pressure is lower (and flow higher). Is this the calculation you want to do?

Mind you I have never worked on a platform, but the head you propose to use for design seems very very low for a 'pipeline'.

To me 'pump discharge static head' has nothing to do with shutoff head. To me 'static' doesn't mean no flow. To me 'discharge static head' is the discharge pressure of the pump in meters of head. More detail please! I am worried that you are filling in a spec sheet and don't really know what you are doing. We want you to do it right.

best wishes,
sshep

 

[Ashjanj]

sshep,

thanks for your comment.

I have requested from the company the pump characteristic of the exiting pump to determine the discharge pressure of the pump.

if i may ask, from your statement "To me 'discharge static head' is the discharge pressure of the pump in meters of head", if i had the pump discharge, can i convert the pressure to head and use it as my Total discharge head for the pump, since it is discharging to the pipeline?

hope to see your response soon.

 

[sshep]

Hello,

I did a little research into what is meant by static discharge head in normal context. Hopefully this is a calculation intermediate (elevation change) to which you are going to add the other more significant "heads" to get a real pump head basis. After more than 20 years of sizing pumps I don't need any help with the calculations, and am therefore not so familiar with the detail to which various texts break these sizing calculations down.

The basis of the elevations used for static head calculation vary. Most of the pump vendors use static discharge head as the elevation change between the pump and the destination irregardless of the pipe routing. This is the proper calculation for the normal operation (but possibly not for start-up) as I described in my post above.

http://www.mcnallyinstitute.com/07-html/7-01.html

Only a few sources use the highest point in the piping as the definition of static head.

http://www.wdmoore.com.au/SelecttheRightSystem/CalculateyourTDH/tabid/63/Default.aspx

I am pretty sure the distinction is related to normal service vs start-up as I described. The trap of using the highest point is that you don't really need the full flow during the packing of the line, so using the design flow based on pressure of the highest point in the piping system might result in a transfer pump with more head than you need or other problems. This is not your case however because a pipeline pump will likely be much higher pressure than simple elevation change! I expect that your pump will be sized for considerable friction losses and other discharge pressure basis which will dwarf the "static head" which you have calculated.

Getting the details of the existing pump is a great check for the replacement. I was afraid you were going to spec something way too small when I saw your numbers. The unfortunate truth in real world engineering is that you are basically on your own and lucky to get anyone to really check your work. The best check available in most cases is the "funny look" check, but this is only available to engineers (and peers) who have experience- i.e. the heads you calculated had a "funny look" to me in that they seemed insignificant relative to what was expected for an actual pump discharge head in the service described.

best wishes always,
sshep