How often should a boiler be blown down, and how should it be done? It seems as if everybody’s mechanic has a different view. Some say once a day. Some say twice a day. Some say low pressure. Some say high pressure. Some say the blow-down should last for a few seconds while others say it should last about eight seconds to ten seconds. Some say the blow-down should be intermittent while others say one continuous blow-down is enough.
How should the boiler feed water treatment compound be introduced? Most people pour it into the condensate receiving tank so the pump can send it to the boiler. But what happens to the pump’s impellers and the condensate receiving tank that is not made of stainless steel or heavy fiberglass?
How can “siphoning” be avoided? This condition occurs when a vacuum is created within the boiler. This vacuum will pull all the water from the boiler, its steam header and take-off lines. This over-filling and siphoning causes water accumulation in the pressing machines and other steam-using equipment in addition to draining the boiler’s water content. As a result, the boiler must be refilled with water (city make-up and return condensate) to its operating level and regenerate enough steam to reach operating pressure from a cold start. This causes maximum delay time in resuming production. It can also cause a heart attack for the owner trying to get the work out.
Usually, the boiler water level control malfunctions and gives a false reading on the water level sight glass which actuates the return pump for replenishing the water supply when the boiler is actually at its proper water level. The false reading of needing more water also turns off the burner causing even more water accumulation in the boiler. The boiler calls for water when it does not need it, and the sight glass shows a normal operating water level when the boiler is filled to the top: vacuum is created for siphoning immediately.
The problem of siphoning sometimes occurs in a tubeless boiler which uses a water level control containing three metal rods used as probes for each of the tri-cock valves. In back of the steam outlet of the boiler’s water level control is a steel baffle with very small openings located a couple inches in from the boiler’s outer shell. When scale and corrosion block these small openings, the water in the gauge glass drops and the metal probe turns on the return condensate pump when the boiler is actually at its normal operating water level. The vacuum created results in siphoning all the water from the boiler into the plant causing flooding of the steam header and steam-fed equipment.
The above problem in a tubeless boiler is corrected by removing the entire water level control assembly from the boiler shell, which will reveal the corroded steel baffle, and drilling a ½ inch hole in the steel baffle.
Aside from properly maintaining the boiler’s water level control and keeping it free of scale and corrosion, a “vacuum breaker” should be installed at the top of a vertical boiler off the steam outlet and at the side (as a take-off) of the steam outlet pipe of a horizontal boiler. A horizontal check valve with teflon seat and rated for 200 WSP (3/4 inch to 1 inch, depending upon the size of the boiler) installed in reverse flow makes a good vacuum breaker. When in normal boiler operation, the swing latch is closed, but when vacuum occurs, or when the boiler is turned off and no more steam is in the boiler, the latch is drawn open.
Detergent or oil in the boiler can also cause foaming, priming and siphoning.
When this condition occurs, the only remedy is to drain and flush the boiler thoroughly. Make sure that the container holding the compound has no detergent or foreign substances dropped into it. I have seen boilers contaminated from leaking still steam coils which discharged NVR into the return line and finally settled into the condensate return tank. Therefore, it may be good practice to install a “sampling valve” on the discharge line of the still’s steam trap. A mere pin hole in the coil or lower wall of the still can contaminate the trap’s condensate discharge. If there is a leak in the coil or wall, you can catch the condensate from the sampling valve and dispose of it similar to your contact water (if on perchloroethylene) or otherwise dispose of it as with the alternate solvent waste until the leak is repaired or coil replaced.
WHEN TO BLOW-DOWN THE BOILER AND HOW OFTEN
It is better to blow-down the boiler twice per day. The first blow down, in the morning, is while steam pressure range is between 20 psig and 30 psig. The second blow-down is in the afternoon after the boiler has turned off and the steam pressure is between 50 psig and 60 psig. At this time the water level control and the water level gauge glass are also blown-down.
HOW THE BOILER SHOULD BE BLOWN-DOWN
A boiler should be equipped with an “angle-stem” blow-down globe valve rated at a minimum of 125 WSP (working steam pressure) to 200 WSP depending upon the steam pressure rating of the boiler. Blow-down the boiler in the following sequence:
- Turn off the burner. For an electric boiler leave the switch on.
- Do not turn off the condensate return pump.
- Open the angle-stem globe valve (nearest the boiler).
- Open the quick-opening gate valve for about 10 seconds, then close it.
- Open the quick-opening gate valve again for about 10 seconds, then close it.
- Close the angle-stem globe valve.
- Open the quick-opening gate valve for a few seconds to clear the line between both blow-down valves, then close it.
- For “probe-type” boiler water level control:
(a ) Open the drain valve to the gauge glass.
(b) Open the quick-opening gate valve for five seconds, then close it, then close the drain valve.
(c) Open the blow-down valve to the water level control.
(d) Open the quick-opening gate valve for about 6 seconds, then close it.
(e) Close the water level blow down control valve.
(f) Open the quick-opening gate valve for a few seconds to clear the line, then close it. - For McDonnell-Miller boiler water level control, the above steps are the same except for opening of the spring-loaded blow-down valve for 6 seconds.
Make sure that the condensate return pump has replenished the lost boiler water to the proper operating level of ½ the distance of the gauge glass.
HOW TO ADD BOILER FEED WATER TREATMENT (COMPOUND)
Boiler water treatment compounds are very alkaline. They are damaging to brass pump impellers when heated and combined with minerals and chemicals found in ordinary city water and most well water. These boiler feed water treatment compounds will cause iron and steel erosion if they are not used to treat the adverse condition of water hardness, pH, oxygen, CO2, etc. The condensate receiving tank does not build up scale, pitting and corrosion as does the steam-generating boiler, and there is no blow-down provision in the tank to check the proper pH and quantity of alkalinity as in the steam boiler.
Therefore, the addition of boiler feed water treatment compound should bypass the condensate return tank and pump, and it should be injected either directly into the boiler or into the discharge side of the condensate return pump.
A professional feed water compound supplier can furnish a pump to discharge directly into the boiler. If such a pump is not available, you can fabricate a workable compound feeder as in the following illustration:
Remember that your boiler is the heart of your plant; the steam it produces is the source of life to almost all of your equipment. Only through care and good maintenance of your boiler and its components will it continue to work for you during the many years of its life. I well-maintained boiler will outlive its owner.
