Archive for the ‘Reducing valves’ Category


These valves are used to reduce the high pressure in a cylinder of gas down to a more safe level.
In the case of oxygen, the pressure is reduced from 2000 psi. to about 55 or 60 psi.

Checking Reducing valves.
When testing reducing valves, I am referring to the British Oxygen type reducing valves.
You will need the following piece of test equipment, most of it you will have to make yourself. The parts for it can be got from a supplier of plumbing parts.
A pressure gauge on a `T’ piece, one end of the `T’ screws into the outlet of the reducing valve and the other must have a tap on it so that you can start and stop the flow, screwed into this tap there is a plug with a hole in it, the size of this hole is important as it offers a restriction to the gas flowing out of the tap, the hole size should be 0.7 of a mm.
The pressure gauge is on the arm of the `T’ piece and should be able to read up to about 100 psi.
What you are testing for is the pressure that the reducing valve is set to when there is no flow through it, this should be about 60 psi -2 or +1 psi, but maybe down to 50 psi on some anaesthetic machines.
Next you are checking that the pressure that the reducing valve gives out does not drop significantly when the reducing valve is working, that is gas is flowing through it.
The pressure when the gas is not flowing through it is called the STATIC PRESSURE, and when the gas is flowing is called the DYNAMIC PRESSURE.
As I said before the static pressure should be about 60 psi, now when you open the valve on your test `T’ piece with the reducing valve connected, the gas starts to flow and the reading on the pressure gauge will drop slightly, this dynamic pressure can be up to 8 psi. less, if it is more than this then you will need to service the inside of the regulator.
This involves the valve seating.
You may also need to change the diaphragm, but normally only the valve seat.
Now for some faults found on reducing valves and the likely cures.

a. Leaking gas from the blow off valve. If the STATIC pressure is correct change the main valve seat, you have a problem called `creep’, this means that the pressure is creeping up from the normal and blowing off the safety valve.
This safety valve will be set to blow off at 120 psi or so. (varies from make to make).

b. Gas leaks from the hole in the bonnet of the regulator.
The bonnet is the large piece that sticks out at the front, and is where you adjust the pressure of the reducing valve from, (more of that later). If you get this you will find that the diaphragm is damaged and will need replacing.

c. The reducing valve makes a popping noise when you turn on the flow. This is called `motor boating’, because it is supposed to sound like a motor boat engine.
It may be a very occasional pop or quite a rapid series of pops and over most of the flow range or just when the flow is first turned on.
For this you may need to change the valve seat, but before you do this give the internal brass valve and all the parts around it a good clean and then reassemble and try again, you may have cured it, if not take it apart again and change the valve seat, this should stop it.
This is not serious and could go on for years without causing a problem. If you have no spares or are no confident in attempting the repair leave it alone.
I once had one in the Maternity hospital in Kathmandu. Without spares to hand I decided to leave it alone. If I had attempted a repair and it had gone wrong I would have ended up with no anaesthetic machine rather than a working one that simply made an irritating noise.

Adjusting the pressure on a reducing valve.
To adjust the pressure on a reducing valve if it is needed, prize the label off the nose of the bonnet, and use a hexagonal key to adjust, your pressure gauge being connected to the outlet, the same set up as you were using for testing the static and dynamic pressure.
Don’t forget to bleed off some pressure if you are adjusting the pressure downwards.
To change the seat on a reducing valve.
Remove the reducing valve from the anaesthetic machine or cylinder.

a. Hold the valve in a vice, taking care not to damage it by using brass or aluminium inserts to your vice jaws.

b. Remove the bonnet from the reducing valve, some are round and very difficult to grip and some have spanner flats on, which is a great help.
On those that have the spanner flats the removal is quite easy. Those that are round can be very difficult unless you have the correct tool, what you need is a strap wrench. This is a tool that has a strap and a handle, the strap is put around the object and tightened up, the handle is then used to turn the whole thing, the greater the force you put on the handle the tighter the strap gets.

Sometimes the strap will still skid around, in this case you can put some rubber underneath the strap to help it grip. If you do not have a strap wrench, you can use a self grip wrench such as Stilton, these will cause a lot of damage to the brass of the bonnet so you must put cloth or rubber around the bonnet to protect the metal.

c. Do not lose the spring and spring centre, also note which way round they go in the bonnet.

d. Having removed the bonnet, you should find that the diaphragm has come away with it, if not remove the diaphragm from the body of the reducing valve.

e. With an AF socket or deep offset ring spanner remove the valve housing you see in the body of the valve.
This will probably be very tight so it is important that you grip the reducing valve body well, some of the very tight ones will be almost impossible to hold properly in the vice and if you think that you will be doing them quite often it will be worth your while to make a special holder to grip the body.
As you lift it out take care not to lose the valve, valve pin, spring and spring retainer.

f. Remove the body of the reducing valve from the vice, and replace it with the valve housing with the large screwdriver slot facing you.
You will now have to unscrew this part, it is called the valve seat retainer. ┬áTo do this you will have to make a blade to fit it as you probably won’t have a screwdriver with a blade big enough in your toolbox.

g. Having unscrewed it, remove it and place it to one side.

h. Remove the valve seat. This may be found inside the valve housing or stuck to the valve seat retainer. If it is in a very bad condition you may have to dig it out with a screwdriver. Having done this make sure that the inside of the housing is quite clean.

i. Put a new valve seat in, and screw in the seat retainer, but do not do it up fully at this point.

j. Refit the valve and valve pin, make sure that the pin is central, remove it and now do up the
seat retainer fully.

k. Refit the spring and spring retainer and screw it back into the body of the valve. Refit the
diaphragm, spring, spring centre and diaphragm gasket.

The diaphragm gasket is sometimes called a skid washer.
The diaphragm gasket or skid washer goes between the diaphragm and the inside of the bonnet.
Before screwing the bonnet back into place make sure that the spring and spring centre(nos.28 and 29 on fig 10) are in place, with the spring centre going in first.
If you have to replace the diaphragm then you follow the same procedure but without removing the valve housing.

Repairs to the pressure relief valve.
On the newest type of British Oxygen regulators, the pressure relief valve or PRV is all made as one unit so that you cannot take it apart, therefore if you have to replace it must be done as a complete unit.
The older ones could be taken apart and the valve seat replaced if it is found to be damaged.
It should be remembered that if you have gas leaking out of here that it often will indicate a problem inside the regulator, most often with the main valve seat.
Attend to this first otherwise you will never cure the leak.
When undoing the pressure relief valve it is best to use a socket or a ring spanner, this is because they are often in very tightly and if you use an open-ended spanner you are putting an uneven strain on the valve and stand the chance of collapsing it.

Broken gauge glass.(not on the anaesthetic machine fitted type).
This can be repaired by cutting a new one from any thin piece of transparent plastic.

Leaking around the bullnose of reducing valves connected to a cylinder.
This is caused by the `O’ ring becoming old and hard.
Cut off the old `O’ ring and replace it with a new one.

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