Unexpected Injector Failure Cause

[Steve Goodbody]

Hi all,

Hopefully the following may be of use to someone experiencing similar symptoms with their injectors….

I was testing some new injectors recently using the traction engine as a testbed. My plan was to install an injector, see if it would repeatedly pick up around 50psi and then raise boiler pressure to 120psi to see if it would still repeatedly pick up at the higher pressure. I would then leave the injector running to flatten the boiler and see at what pressure it knocked off. I’d then replace it with the next injector and repeat the test. It seemed like a good plan.

Installing the first injector, all was fine at low pressure. At high pressure the injector picked up exactly once. On shutting off to repeat, steam and water droplets would then spit from the overflow and I could not get any water through to cool and restart it. Running water over the body solved the problem temporarily, but the same issue would recur as soon as the injector was again shut off at high pressure. Suspecting a leaking boiler check-valve I decided to continue the test and let the injector feed until it knocked off due to low pressure. At least that first injector seemed fine.

I then repeated the test for the two other injectors and exactly the same thing happened in each case. All was fine at low pressure but, at high pressure, each injector would work exactly once and then spit steam and not pick up unless cooled with a water dump over the body.
Although happy that all three injectors had worked, and convinced that the problem was a leaking boiler check-valve, I decided to increase pressure one more time to confirm.

Starting then stopping the injector resulted in the same symptoms once again – steam and water droplets spitting from the overflow and a too-hot injector to restart. To confirm the problem was the boiler check I then disconnected the injector from the boiler feed pipe expecting to see steam coming back down the feed pipe. I was very surprised to find steam still spitting out of the injector but nothing at all from the feed pipe. I was completely wrong - the problem was the injector steam valve.

In conclusion, once stripped down, it turned out that the injector steam valve would seal perfectly at lower pressure but, due to a tiny imperfection on the spindle nose, it wouldn't fully re-seal when opened and closed at higher pressure.

I suspect this small steam leak was sufficient to heat the injector to the boiling point of the feedwater and cause a vapor-lock in the water feed. The small steam valve leak was the root cause of the hot injector and that leak, combined with a little bit of feedwater boiling in the injector body, was the cause of the spitting at the overflow and, to me, looked exactly like a leaking boiler check valve.

I hope the above is of help to someone!

Best regards
Steve

[hoppercar]

i had the same problem, except it wasn't with globe valves, it was with the quick pull steam valves. had rubber o ring, kept coming apart, and plugging up the injector.....now the other one is doing the same thing......strands of rubber, plug up the steam cone

[Fred_V]

I have had home made inj steam valves with orings on the stem and never had that problem. In 25 years of running I had one crack and leak. I replace them every 2 or 3 years.
What kind of oring are you using? I just used the black ones.

[Bill Shields]

unfortunately, ''black' describes a wide range of elastomers...some of which are OK for steam, others are usless.

[Fred_V]

Buna N

[Bill Shields]

probably not the best for steam service.

Suggest viton as a minimum

[Fred_V]

Some people argue against that too. I have no complaints about what I use.

[hoppercar]

these are supposed to be viton, but they shred and come apart...

[RET]

Hi,

I have a stock of Viton "O" rings and they are an orangy brown in colour, not black. Remember, rubber expands a lot as the temperature rises and you need to take this into account. Also, if the "O" ring "pocket" is too tight, it would behave the way you describe.

For instance, if I use an "O" ring as a piston ring, I cut the groove so the "O" ring just touches on both sides and make the depth of the groove so that there is only .004 to .007 ins. of radial interference. Remember, in service, "O" rings are "self energising." Once the seal is made, the pressure causes the "O" ring to deform to further seal against any leakage.

Do NOT install "O" rings the way it is described in the "Parker" "O" Ring handbook! Even for a static seal, the interference is too great. The dimensions given might be acceptable for a stationary seal for high pressure hydraulic service at ambient temperature, but not the way we use them.

Just what I've learned.

Richard Trounce.

[Bill Shields]

brown is an odd viton color.

all mine are black as a dark room....

perhaps they can be ordered in different colors...????

[NP317]

My new selection of "Viton 75" o-rings are all brown in color.
The 75 refers to durometer.
RN

[Bill Shields]

looking in catalogs, i see brown viton listed....but darned if I have ever seen or used one...