Mechanical water pumps

[ivanj]

On the drawing for my King it shows a water pump having an o ring at the end of the plunger. I think this is not a particularly good idea as normal o rings are not designed for reciprocating duty - I know that there ones that are. So what i have done is make the plunger and bore a very close lapped fit and maybee I will fit a gland on the outboard end. Is there any problem with doing it this way and does water provide any lubrication between phosphor bronze and stainless steel? The piston speed will be slow as it is operated from the driving wheel axle so 2 to 3 Hz is going to be top speed. Do I even need a gland at the outboard end if I am prepared to accept the occasional drip which will ensure a water seal round the piston?

[alanstepney]

ivanj said:

On the drawing for my King it shows a water pump having an o ring at the end of the plunger. I think this is not a particularly good idea as normal o rings are not designed for reciprocating duty -

Perhaps not but it is commonly done.


know that there ones that are. So what i have done is make the plunger and bore a very close lapped fit and maybee I will fit a gland on the outboard end. Is there any problem with doing it this way

No.

and does water provide any lubrication between phosphor bronze and stainless steel?

Yes.

Do I even need a gland at the outboard end...?

Yes.

[waggy]

Ivan,

As Alan correctly says, yes you do need to seal one end of the ram. If you don't there will be water water everywhere!
The only way you MIGHT get away with a close fitting ram is to mount the pump in the water tank and keep the level above the pump at all times. This would stop the spray of water when it leaks past the ram and you wouldn't get wet. The delivery of the pump would have to be capable of coping with the bypass and filling the boiler. The stirrup pump I and many others used for pressure testing the coils in carriage heating boilers (flash steamers) leaked no matter what we did, the barrel was out of round and our boss wouldn't buy a new one! The only way to stop getting wet was to stand it in a barrel with the top cut off and full of water. Pump like mad and you could maintain test pressure. (150 psi).
Methinks a seal is the solution. Oink oink!

Waggy.

[simonwass]

Whats the difference between a static o ring with a plunger operating inside it and a moving o ring inside the bore?
There will be the same moving action in terms of ft/min, its just acting on one side or the other.
I have never come across info stating that o rings should always be stationary? So long as for moving seals using o rings that you provide a groove which allows the o ring to roll, it'll be fine. The Reeves book used to have tables for o ring tollerances, but I'm sure they'll be online somewhere.
A plus point of using a moving o ring inside a pump barrel is that you have no gland nut to make so the pump will be less bulky. Also it wouldnt do justice to a King to put a water pump on it!

[Tel]

cut the groove and fill it with twisted teflon tape

[ivanj]

Thanks fellas, a good haul of thought provoking replies. I like the idea of Teflon as it does have good lubricating properties. Simon, the water pump is between the frames and called for on the drawings, the engine also carries an injector so it will not show. What I will do i think is to make an outboard gland packed with teflon tape with maybe the groove and tape on the business end.

[freddo]

Ivan, why not follow British Aerospace and use a labyrinth seal - mulltiple grooves in the piston, and as long as they have sharp edges, you need no seals. The idea is the pressure drop across each "labyrinth" is progressive, and with maybe 5 grooves in the piston (cut them with a 1/16" parting tool), it works like a charm for water pumps.

[redmog]

I like the above idea posted by Freddo. Turn the grooves first before the final skim leaving sharp edges that should do no halm. So could this be used for the pistons in the cylinders or is the thermodynamics of steam different? What shall I use in my cylinders made from Reeves cast gunmetal - rings - graphite yarn - or 'O' rings, and what should the pistons be made from. Advice welcome, it's a 31/2" Bantam Cock.

[Noddy]

Thanks Freddo. The principle really sound, and is used to seal the gas pistons on gas operated machine guns rifles etc, where pressures are in thousands of PSI, and reliable free movement is essential.

The cut outs also serve to keep any crud out of the way of the sliding surfaces.

If appearence doesn't matter, but long term reliability does, the principle of sludge pumps from mines could be used.

In the sludge pump, the piston and bore are only ever in contact with lubricating oil. The oil which it displaces then acts as the piston in the top of a seperate cylinder with one way valves at the bottom. It sucks in and pumps out abrasive sludge, and potentially pumps it up to 4km vertically out of some of the deepest mines

[steam4ian]

G'day all,

Judging by what I read in a paper presented by Sulzer, labyrinth seals are used in some oilfree compressors. They do work but appear as if they need the piston held away from the cylinder surface by extenal guides.

Equally in ouir applications the grooves could hold oil which could do the sealing.

Regards,
Ian

[simonwass]

freddo said:

Ivan, why not follow British Aerospace and use a labyrinth seal - multiple grooves in the piston, and as long as they have sharp edges, you need no seals. The idea is the pressure drop across each "labyrinth" is progressive, and with maybe 5 grooves in the piston (cut them with a 1/16" parting tool), it works like a charm for water pumps.

Our turbines have these 'seals', they go from 165bars to atmosphere in about 10 knife edges. I still cant get my head around the idea! I know how it works, I know it works but I dont get why it works!

[ivanj]

Turbine seals have leak off points which go into the LP system but the labyrinth seal certainly works

[freddo]

Ivan, I've used the labyrinth seal on several axle-driven water pumps on locos, and with a stainless piston in a gunmetal bore, they are totally maintenance free - I can thoroughly recommend that idea.

[Shawki Shlemon]

Hi
I build the King and I did fit the pump and injector as per the plans.Back to the pump ,if you add gland at the end you will end with short eccentric rod and too much up and down forces on the ram . For Labyrinth seal you will need a number of grooves and therefore longer ram and the same problem as above and the up and down forces will not help the seal unlike the turbine .O ring has been used successfully for long time ,my suggestion is stick to it.

[Tel]

By using the teflon tape in the groove in the pump ram, you will not need a packing gland at the back. 'O' rings, in my experience, are a bit unpredictable - you either get the 'right' fit or you miss it.

The teflon tape is more forgiving.