Feed water pump best practices

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Glenn Brooks
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Joined: Mon Nov 10, 2014 1:39 pm
Location: Woodinville, Washington

Feed water pump best practices

Post by Glenn Brooks »

Hello all,

Thanks much for the replies to my water pressure gauge question.

I have decided to reroute my existing water pump feedwater piping to locate the needle valve control in the cab. This will require replacing most all of the original system. I am wondering if there is some basic design principle, or best practice, regarding diameters and location of the new intake, return, and high pressure feed lines to the boiler?

Unfortunately I can’t find any notations for best practice for the different diameters of piping, or where exactly the return line should diverge from the intake or pump. Or even if the feed line and return line diameters should differ from the hi pressure injection line...

As FYI, my existing design is on a 12” gauge Ottaway, having a 12” boiler and simple 1/2” dia plunger type pump- driven mechanically off a lever attached to the cross head.

Here are the key metrics:

The intake line, from the tender to the pump, is 3/8” copper.
The high pressure output line from pump to boiler is 1/8” pipe.
The by pass line, diverges from the 1/8” high pressure boiler feed line a couple of inches above the pump, and leads to the the needle valve via 1/4” diameter copper.

Also, the pump outlet is 1/8”.

Hence, it appears the water flow through the low pressure intake line 3/8” dia = (1/8”hi pressure feed + 1/4” variable rate low pressure bypass).

Does this make sense from a flow dynamics viewpoint? If not, how should the diameter of piping be different at the various segments in the loop?

Second question. Does it matter where the return line diverges from the feed water, upstream of the pump?

Final question, what is the ideal size line out of the pump? Is there some ratio of input size to output size? Or an optimum ratio of pump diameter to output size?? E.g. does the hi pressure output diameter depend in some way on the pump diameter or capacity?

Sorry for all the questions - trying to understand what is the best practice time for re- constructing the existing bits and pieces...

Thanks much,
Glenn
Moderator - Grand Scale Forum

Motive power : 1902 A.S.Campbell 4-4-0 American - 12 5/8" gauge, 1955 Ottaway 4-4-0 American 12" gauge

Ahaha, Retirement: the good life - drifting endlessly on a Sea of projects....
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ChipsAhoy
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Location: Arizona

Re: Feed water pump best practices

Post by ChipsAhoy »

Glenn:
On your second question. I assume that you mean...where is the best place for the return line (which originates from the discharge of the pump) to be plumbed to.
If that is the case, on some of our closed systems in the hydrocarbon industry, any "relief valve" discharge (which is what you are doing), was routed upstream as far away from the pump suction as practical and definately upstream of the suction filter. As a pump may wear, a short loop would cause contaminants in the suction feed to rapidly increase. Whereas, if the discharge were returned upstream of the pump suction it would most likely go thru a suction filter. Our suction filters have a differential pressure reading which indicates a "plugging" filter. I wouldn't suggest adding such a guaging system on a locomotive, it could cause more trouble than it is worth. But, returning upstream of your filter should be a good idea.
This suggestion may be way beyond your question.
Glenn Brooks
Posts: 2930
Joined: Mon Nov 10, 2014 1:39 pm
Location: Woodinville, Washington

Re: Feed water pump best practices

Post by Glenn Brooks »

Chips ahoy, thanks very much. I can definitely route the bypass as far away from the pump as possible, couple of feet probably, and possibly plumb it back in on the far side of the filter.

Any idea if the diameters of the pipe make any difference, anywhere in the system?
Again, thanks much.

Glenn
Moderator - Grand Scale Forum

Motive power : 1902 A.S.Campbell 4-4-0 American - 12 5/8" gauge, 1955 Ottaway 4-4-0 American 12" gauge

Ahaha, Retirement: the good life - drifting endlessly on a Sea of projects....
Pontiacguy1
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Joined: Thu Apr 26, 2012 10:15 am
Location: Tennessee, USA

Re: Feed water pump best practices

Post by Pontiacguy1 »

Question: Can you feel your pump surge when it isn't pumping against boiler pressure? If you can, then you could probably use a larger return line, because there is still some amount of restriction there that is robbing power. You didn't say whether or not your return line dumped back into the inlet side of the pump or back to the tender.
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Fender
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Location: Chattanooga TN

Re: Feed water pump best practices

Post by Fender »

I would suggest running the return line to the tender. Will be easier to prime the pump if it becomes “air-locked” than the configuration where the return line feeds directly to the inlet.
Dan Watson
Chattanooga, TN
Glenn Brooks
Posts: 2930
Joined: Mon Nov 10, 2014 1:39 pm
Location: Woodinville, Washington

Re: Feed water pump best practices

Post by Glenn Brooks »

Pontiacguy1,

So far I don’t have any operating experience with the pump, as it hasn’t been connected since I’ve owned the loco. So, performance wise i don’t yet know how the pump reacts during operation. I do know the existing return line is half again as large as the hi pressure injection line (1/4” V. 1/8” diameter).

The original piping returned back to the inlet side, right behind the pump. It was set up in a very short loop in the area in front of the drivers - maybe 12” or 18” of piping - around 4-6” on each leg. My intention is to re-route the return line back thru the cab, so I can control the needle valve without stopping the loco and walking around to the front of the boiler to adjust flow rate- then return flow on to the tender outlet.

Glenn
Moderator - Grand Scale Forum

Motive power : 1902 A.S.Campbell 4-4-0 American - 12 5/8" gauge, 1955 Ottaway 4-4-0 American 12" gauge

Ahaha, Retirement: the good life - drifting endlessly on a Sea of projects....
Pontiacguy1
Posts: 1566
Joined: Thu Apr 26, 2012 10:15 am
Location: Tennessee, USA

Re: Feed water pump best practices

Post by Pontiacguy1 »

I would suggest that you use the same size line for your return as the water feed line that goes into the boiler check valve. I've run locomotives before where the pump could be felt still forcibly pumping water (surging) when the bypass valve was open completely, even at low speed. That usually means there is some sort of restriction in the return line, bypass valve, pump check, or something. I also would suggest running the line back to the tender. It will then prime by itself. Also, I have seen some people put the water return line up high in the tender, and have a small door right over it, so that they can see if the pump is actually moving water or not. I thought that was a great idea, one that I will probably use in the future.

If you don't want to run it back to the tender, then put a Tee in the line between your bypass shutoff valve and the pump itself. Put a valve on there so that you can close your bypass valve, open up the dump valve, and let the pump prime itself.
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