Best rpm for plunger pumps?

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Best rpm for plunger pumps?

Post by Steam Captain » Sat Mar 06, 2021 12:12 pm

Yello,

I've been brainstorming about buying a pump from ebay or building one from brass fittings. I can't decide yet, but I'd actually like to build one for the experience.
I have books about model steam engines and it says a plunger pump shouldn't run higher than some 500 rpm, otherwise the valves wouldn't respond properly anymore.

That made me wonder if there is any practical experience or a general rule of thumb what rpm range a plunger pump for launch-sized steam engines are best suited to run at. Right now, I'm running numbers for a pump with 60rpm. But the small rpm is making me wonder if the performance of ball-type check valves would suffer or if there is a lower rpm end for ball-type valves in pumps.

I guess I subconsciously aim the question at Mr. Rosse, as he seems to have profound knowledge about fluid dynamics, but of course I'll be grateful for every input.
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Re: Best rpm for plunger pumps?

Post by barts » Sat Mar 06, 2021 4:51 pm

There really isn't any hard and fast rule, except that fast is hard :).

If you look at a Hypro dual piston plunger pump, you'll see that the valves are very lightweight so they can run at 1800 rpm driven directly by an electric motor. This makes these valves subject to easier clogging with sticky oil, debris, etc; in their usual service for pressure washing, the inlet is pressurized to whatever city water pressure might be so simply unloading the pump will make them tend to clear the valves.

Since in our small steamboat applications we only have a few inches of water pressure (or vacuum), the springs on the inlet checks need to be fairly light to keep the pump somewhat self-priming/purging.
These light springs will mean that the design of the check needs to rely on the water flow in the reverse direction to close quickly at high speed, which makes the pump less effective and easier to hang up.

My idea of an ideal steamboat boiler feed pump is a geared down displacement dual plunger pump running at perhaps 100 rpm max, and slower preferred. The check valves would be proper brass/bronze lift checks installed so gravity helps close the valves, with Allen head lift stops drilled and tapped into the top of the valves so that the stroke of the checks can be limited if needed at higher speeds. The Teflon packing would be adjustable while running, and I'd be able to unload the pump to clear it while running with either a spring return valve or if reachable simply by depressing the float in the hot well. There would be a easily cleaned fine mesh filter ahead of the pump to keep pine needles and other impediments to proper operation out of the check valves. I would size this pump at about 2.2 x the needed flow, so that if one side got hung up at a critical juncture I could keep steaming until it was safe to stop and clear the check. A pressure relief valve should be plumbed in before any shut-off valves, and the relief outlet should be readily visible to the engineer if someone manages to close the boiler feed valve.

- Bart
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Re: Best rpm for plunger pumps?

Post by fredrosse » Sat Mar 06, 2021 7:21 pm

The issues with reciprocating pump speed are very much complicated by several variables. The technical issues are generally to find a balance between suction check valve weight/inertia, which tends to impair rapid opening/closing, vs. the valve closure force, which fights against the valve being opened generally with only atmospheric pressure available to force the suction valve open during the suction stroke.

For example, pressure washer pumps, running at typical 3600 RPM work well, as they have garden hose house pressure feeding the suction. This allows fairly stiff suction check valve springs, which can therefore close the suction check valve rapidly enough to allow such a high RPM. Small mass check valve clappers also helps here, and probably would not function with much larger piston/plunger displacement.

On the other end of the spectrum, my hand pump (60 strokes per minute +/-) uses common plumbing swing check valves, where the valve closure at the end of the suction stroke is only due to gravity acting on the swing check, and the swing assembly is fairly massive compared to thin disc type valve elements. This is entirely satisfactory because of the relatively slow stroke times here.



In between these extremes is where typical engine driven steamboat plants operate, the engines typically running at a few hundred RPM, and the machinist made pump often using metal ball check valves, seated with moderate springs on both the suction and discharge check valves. The technical details here can get very complicated, but there are a few points to consider:

1. The discharge check valve can be a stainless steel ball, spring loaded to closure, with discharge area enough to keep water velocity below 50 feet per second. These valves usually function well, as there is plenty of fluid pressure force to open them on the discharge stroke. The spring causes extra pressure drop to allow the valve to open, and this differential pressure should be kept below 15-20 PSID (pounds per square inch differential)


2. The suction check valve should be designed with less than 5 PSID required to lift the check disc, or ball, off its seat, thus a light spring is warranted. This is counter to the spring’s ability to reseat the suction valve quickly at higher RPM, and is improved by using a light weight check disc or ball. Suction velocity through the suction stroke should be lower, around 10 ft/sec is OK. Ideally the suction check could look like a typical leaf type check valve, as found in typical air compressors, but accommodating this arrangement is often difficult to make.

3. For practical reasons, ball type suction valves are used on small steam plant feed pumps, and a metal ball is often too heavy, due to its inertia, to function well at several hundred RPM. A heavy spring cannot solve this problem, as only atmospheric pressure is available to open the suction valve.

4. I have found that these ball check issues can be greatly reduced by using Teflon balls, about 150% - 200% of the discharge ball diameter. Teflon weight is far less than metal, yet makes a good ball check seal up to several hundred PSI discharge pressure.

5. As to the springs used, one typically needs a very light duty spring, and finding these in corrosion resistant material is often difficult. I have found hard brass wire works OK, winding springs on a shaft mounted in the lathe. Better yet, and often available to we steamboaters, is the winding of springs with typical silver solder wire. This wire winds well, and with several coil turns gives a light spring force. Less turns gives a stiffer spring. Larger wire diameter gives a stiffer spring. Smaller major coil diameter gives a stiffer spring. One can fairly easily experiment with different spring winds to get a steam plant feedwater pump to run satisfactorily at several hundred RPM, without breaking the bank with revisions.
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Re: Best rpm for plunger pumps?

Post by dampfspieler » Sat Mar 06, 2021 9:12 pm

Hi,

i had ball valves at the direct driven feed pump of my steam lorry. It runs up to 1.400 rpm. The balls destroyed their seats.
PoP_SaugVentWaPu001kl.jpg
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So i have built "mushroom valves" with light phosphor bronze springs.

I have also attached an air vessel after the discharge vale. Now the pump works well up to 1.400 rpm for years.

Best Dietrich
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Re: Best rpm for plunger pumps?

Post by DetroiTug » Sun Mar 07, 2021 4:47 am

I use Apollo soft seat check valves designed for hydraulic service. No problems with them.

Some of the steam car guys use ball checks and they report that lightweight balls made from Delrin work very well.

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Re: Best rpm for plunger pumps?

Post by Steam Captain » Mon Mar 08, 2021 1:30 am

This information will get me very far. Is there a valve type better suited to obtain a low opening pressure loss? I blueprinting an engine that keeps me capable to experiment with both exhaust modes - exhaust into the atmosphere and into a condenser. That could well mean an absolute pressure of 0.1 bar / 1.45 psi at the pump suction side. It is not easy to prevent too much suction, but I've solved the cross section dimensioning and the only lose end would be the pressure loss due to the inlet check valve. 1.45 psi is not much to work with.

Looking around on the web, I found the term "opening pressure" I understood it as pressure loss through the valve. I found commercial options with 0.1 bar and even 20 mbar opening pressure, which is perfect for me. But building it would be fine, too. I guess now that the theory is roughly clear, the question is what type of inlet check valve is in use in condensing steam plants. That way, I wouldn't need to invent the wheel anew and just have a reference of what to look out for. Many engines use pumps driven directly from the crosshead and hypro pumps (I can only find the centrifugal type on the market) or pressure washer pumps. Mike from the Puget Sound uses a Hypro pump plus a keel condenser, if i remember correctly. And I usually see condensers plumbed into the system with crosshead-driven pumps. So at least those two seem to be capable of having a very low suction pressure (suction head) without evaporating the water.

Quick note: The pump is supposed to create 50 bar / 735 psi. I don't have the knowledge about that, but it might reduce the choice of check valves.

Uh, the torment of having a choice. I'm really undecided about this one. What would you do?
the arduino version steam engine indicator: https://app.box.com/s/b2i0z3gw6ny3rcfdet5xjg8ubrfu799i - app version coming soon
Excuse my occasional long response time. It's caused by the side effects from ptsd.
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Re: Best rpm for plunger pumps?

Post by fredrosse » Mon Mar 08, 2021 8:29 am

There are generally two pumps needed for a condensing plant that runs with vacuum exhaust.

The condenser runs at vacuum, and "wet air pump" is used to bring the condensate out of the condenser, and send this water to the "hotwell storage tank", which is generally an open tank at atmospheric pressure. The "wet air pump" also removes non-condensables (as well as condensate water), such as air from the condenser, which allows forming vacuum engine exhaust conditions. The wet air pump has far greater displacement than the main feed pump, and has only very little pressure available to open the suction valve on the suction stroke, only a small fraction of atmospheric pressure. Common suction and discharge valves here can be made with a thin (1/16 inch, 1.5mm) rubber sheet flapper, covering several small holes (1/4 inch - 6mm diameter). My wet vacuum pump has 1.5 inch bore x 2-1/2 inch stroke, and has 12 holes drilled in the pump piston, arranged around the pump rod, 5/8 inch diameter. A rubber sheet cut 1-3/8 diameter slips on the pump rod, and seats itself on top of these discharge holes. This pump only needs to work against atmospheric pressure discharge, and a similar arrangement for the suction valve can work well.

If the plant is running with slightly above atmospheric pressure engine exhaust, then the wet air pump can be eliminated, as feedwater to the boiler feed pump can be found in the hotwell storage tank, forced there with the condenser pressure slightly above atmospheric pressure. Alternately, boiler feed pump can suck from other water storage tanks, or directly from the lake.

The main feedwater pump then takes suction from the hotwell storage tank (or other tank, or the lake), and feeds the boiler at high pressure. This pump needs to be far more robust than a "wet air pump".
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Re: Best rpm for plunger pumps?

Post by barts » Mon Mar 08, 2021 4:00 pm

That's a neat wet air pump, Fred. On the drawing you note that there's discharge on both upstroke and downstroke. I assume the latter is due to the difference in area above and below the piston?

This is a simpler design than the Navy-style pump, and avoids the complications of dragging the piston seals across the center suction ports. In your design does the piston have an o-ring seal, or are those water groves?

Thanks -

- Bart
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Re: Best rpm for plunger pumps?

Post by Steam Captain » Mon Mar 08, 2021 4:36 pm

I thought I was smart and I'd just use one pump for the condenser and the feed pump, but I forgot the condenser pump doesn't deliver 100% liquid water. And the piston area for a vacuum pump would kill the mechanisms if I want to use them against the full boiler pressure.

There are so many variables, I think I just jump into the challenge with the wisdom sprinkled on me in mind.
the arduino version steam engine indicator: https://app.box.com/s/b2i0z3gw6ny3rcfdet5xjg8ubrfu799i - app version coming soon
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Re: Best rpm for plunger pumps?

Post by Lopez Mike » Mon Mar 08, 2021 5:15 pm

Bart gave me his old Hypro because he was too lazy to rebuild it!

I bought some spare valves, cleaned up the pump and it's been running for years with a squirt of grease once a year with no problems.

The spare valves are still in their plastic envelopes. Pretty little things.

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