# Pump head question



## breezy_moto (Mar 24, 2018)

Trying to work through some pump problems as I've identified it as one of my weak points. For this question, from PPI Ch. 18, I'm confused on the head calculation in order to find Power. From the lake to the pump they only take into account the elevation head but not the friction head. According to the solution, total head = h elevation from lake to pump + h elevation pump to tank + h friction pump to tank. Can anyone shed some light? thanks.

https://photos.app.goo.gl/ZzeGTHmhucVsRUo32


----------



## Slay the P.E. (Mar 24, 2018)

breezy_moto said:


> Trying to work through some pump problems as I've identified it as one of my weak points. For this question, from PPI Ch. 18, I'm confused on the head calculation in order to find Power. From the lake to the pump they only take into account the elevation head but not the friction head. According to the solution, total head = h elevation from lake to pump + h elevation pump to tank + h friction pump to tank. Can anyone shed some light? thanks.
> 
> https://photos.app.goo.gl/ZzeGTHmhucVsRUo32


If they used an equivalent length of 7,000 ft to calculate friction head, then they did account for friction everywhere. The 7,000 ft is the equivalent length from the lake to the tank.


----------



## breezy_moto (Mar 24, 2018)

Dammitt, they got me! I need to read more carefully. Thank you.


----------



## DanHalen (Mar 24, 2018)

Another way you might see something like this is K factors. They're used to calculate frictional headloss with fittings. 

Sent from my Pixel 2 using Tapatalk


----------



## pat_in_the_hat (Mar 25, 2018)

i have another pump question. not to hijack your thread but it seems you got your question answered

Question 138 in the NCEES latest practice exam.

They give inlet pressure (20-psig) of the pump. a discharge elevation (+160') and pipe friction loss (20'). And they are asking for the "minimum pressure rating (psi) of the piping system).

The way I did it was convert everything to psi, sum the elevation and friction loss since that is working against the pump, then subtract the inlet pressure (20psi) since this is helping the pump.

But the solution sums all 3 psi's together. 

Do they do this because they are asking for the pressure rating of the SYSTEM? I just feel like inlet pressure should be negative compared to elevation and friction


----------



## Vel2018 (Mar 25, 2018)

pat_in_the_hat said:


> i have another pump question. not to hijack your thread but it seems you got your question answered
> 
> Question 138 in the NCEES latest practice exam.
> 
> ...


Question 138 in the 2016 NCEES which is the latest is about refrigeration.


----------



## breezy_moto (Mar 26, 2018)

Vel2018 said:


> Question 138 in the 2016 NCEES which is the latest is about refrigeration.


Not from what I see. I believe we're talking about the HVAC &amp; R exam.

The inlet pressure is indeed helping the pump but that in turn adds to the pressure after the pump doesn't it? That's the way I looked at it.


----------



## Vel2018 (Mar 26, 2018)

breezy_moto said:


> Not from what I see. I believe we're talking about the HVAC &amp; R exam.
> 
> The inlet pressure is indeed helping the pump but that in turn adds to the pressure after the pump doesn't it? That's the way I looked at it.


I see. Thought it was TFS.


----------



## Slay the P.E. (Mar 26, 2018)

breezy_moto said:


> Not from what I see. I believe we're talking about the HVAC &amp; R exam.
> 
> The inlet pressure is indeed helping the pump but that in turn adds to the pressure after the pump doesn't it? That's the way I looked at it.


Yes.

What you’re after here is the highest pressure anywhere within the pipe, which would be at the pump discharge for this system. That would be the minimum “rating”.

Try framing the question like this: The gage pressure at the pump discharge is most nearly:


----------

