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[BarryM]

 This concerns a 4HP compound engine but, as the system is identical to that used on many models, I would be happy for any suggestions that might solve a problem currently bothering my brain cell.
The problem lies with the boiler feed system which sometimes delivers and sometimes doesn’t.
The boiler may be fed either by a hand-operated ram-pump or an engine-driven ram pump. Both pumps draw from a hotwell, always providing a positive head, via a common suction line. They then discharge through a common line to a feed heater, a pressure damper and then to the boiler via a lift-type non-return valve and a feed stop valve. Before the non-return valve, a bypass valve is teed-off and permits feed to be recirculated back to the hotwell. Thus, assuming constant engine speed, boiler feed is regulated by the bypass valve.
Using the hand-pump I have confirmed that both the pumps discharge lines are leak-free and the lines to the boiler and hotwell can be primed without problem.
There are no leaks on the suction lines.
The suction and discharge valves (all steel balls) in both feed pumps are free of debris and making good contact with their seats.
The boiler non-return lift-valve is clean, a good fit on the seat and has sufficient clearance to avoid sticking as temperatures rise.
The feed bypass valve is a screw-down, non-return type. All internals are in order and, when the engine is running and the bypass opened, feed can be observed returning to the hotwell. Similarly, if the engine is stopped and the hand pump used with the bypass opened, feed returns to the hotwell. When the bypass valve is closed there is no return to the hotwell.
Mostly the system works as designed but on occasion it doesn’t deliver to the boiler. I have discovered that if both the feed isolating valve on the boiler and the bypass valve are carefully shut in (obviously without closing both off) the system will pick up and the engine-driven pump will deliver to the boiler. The feed isolating valve can then be opened fully, the bypass closed and everything functions normally.  This will continue but sometimes – not every time – after the bypass has been used, the feed delivery problem will surface again.
It seems that, as noted, temporarily restricting the feed discharge increases the back-pressure on the pumps/valves and that overcomes the problem. This probably indicates that something is not seating correctly but what – and why?   
The bypass valve has provided problems in the past but all tests demonstrate it is functioning normally now.
Can anybody suggest a solution?

Barry M

[xrad]

Hi Barry,
1) I would use seperate suction lines from the hot well to the pumps as a safety feature
2) Test pump pressures
3) If everything is working except the bypass valve, then you have your answer

[BarryM]

xrad,
1) I'm not clear how seperate suctions would improve safety or solve this problem. Even if both the engine driven pump and hand pump failed there is still the third option of an injector which operates on an independent circuit.
2) The pump pressure is fine when working correctly. It is the intermittent and unpredictable loss of pressure that is the problem.
3) As noted in my text, there is nothing wrong with the bypass valve. All the valves concerned have been checked out and found OK.
I am focusing on the possibility of a vapour lock in the suction related to pressure changes in the system. This may in in turn be related to some recent pipe run alterations that I have only just been advised of.   I may know more after getting the chance to check over the plant again on Wednesday.
Regards,
Barry M

[xrad]

B,
Vapor lock can be an issue. Post some pics of your piping circuit.  Steam injectors at scale sizes often have issues so having this as a third pumping option may not be a reliable alternative if the first two fail.
What is the elevation of the hot tank draw pipe/fluid level in relation to the pumps?
You could add temporary pressure guage and a one way valve on the common low side of the pumps and another guage on the high side to test pressures.

[Jerry C]

If hotwell provides positive suction head then there can be no vapour lock. If everything works as you say then system will work. As you say I doesn't work then clearly one of your premises is incorrect. If the piston seals/rings are good then a valve has to be passing. The pump inlet valve on pump stroke is closed by boiler pressure plus. On suction stroke valve is opened by atmospheric pressure plus head. I suspect that when boiler pressure is high the inlet valve sticks shut.
Jerry.

[Landlocked]

Have you tried the handpump in parallel with the driven pump?  The feel of the pumping action might provide clues. 


If the hotwell temperature is high enough and the piston speed fast enough on the driven pump, you could still have vapor lock even if the pump is below the water level.  Big steam refers to condensate depression, the amount of subcooling required of the condensate not to flash in the inlet of the condensate pump. 


I doubt if you can pump the hand pump fast enough to flash the condensate so a steady pumping feel will rising level that ceases to rise when you stop pumping might indicate vapor lock.



Ken

[derekwarner]

  ....Barry......if this system has been serviceable for x years without issue ......what has changed?. ......you may have answered your own question
"This may in turn be related to some recent pipe run alterations that I have only just been advised of" .............. Derek

[BarryM]

Gents,
There is no evidence of any mechanical malfunction in the form of dodgy valves although scrutinising the pipe runs more closely is on the agenda for tomorrow. This problem did crop up before the pipes were removed/replaced for boiler repair but very infrequently and so there may be a contributory factor there.
The hand pump is small, slow and insensitive and does not offer any clues. The hotwell take-off is above the inlet to the engine-driven feed pump by about three/four inches depending on trim. The total head is likely to be about 10" - 14" depending on operating conditions.
The hotwell holds about 4 gallons and is usually run with the lid swung open. I have not put a thermometer into it (yet) but from the appearance of the surface and the negligible amount of steam vapour given off, I do not think that hotwell temperature is a factor.
A 'Brains Trust' of assorted steam launch operators at this end is pointing the finger at vapour/air lock in the pump suction and one of them has advised that he has a similar problem which he found he could overcome either by the same method as I described above or by an air vent on the feed pump. His explanation is aerated feed in the hotwell and that strikes a chord with me. The air pump also discharges to the hotwell and the water therein does have a cloudy appearance which may be more than just feed-treatment compound. (As I write these words, I'm wondering if I'm having a small Eureka Moment.)
The launch will be coming out of the water very soon (getting pretty nippy on the Loch) and I will take advantage of the opportunity to consider how/if the pipe runs can be improved and a vent on the main feed pump introduced. Deaerating the feed at source is unlikely and so efforts at improving the pipe venting and opportunity for airlocks to form, appear to offer the best solution.
Thanks for all contributions to this thread. The little grey cells are getting a workout.
Regards,
Barry M