McKenzie’s Boiler Blog

BOILER - LOW WATER

The effect of low water in a boiler could result in, at the least, a leakage and, at the worst, a catastrophic explosion, depending on the severity of water loss and the boiler design. Without the water to surround the crown sheet over the firebox the exposed metal reaches such an extreme temperature that the tensile strength decreases to dangerously low point and fails.

In a fire-tube boiler, a low water condition will expose the tubes to high temperature gases, expanding the tubes to such a great extent that they will break the rolled in seat with the tube sheet. Leaks can be detected as a result of this expansion and may even result in the tubes collapsing.

Regardless of the boiler design, it is obvious that low water is a situation that must be avoided at all costs. Anthony L Kohan, author of “The Boiler Operator’s Guide”, recommends the following procedure “if water is not visible in the gauge glass because of failure of the feedwater supply…”

1. Shut off fuel to the burners.
2. Check the water level by trying the try cocks and water-column drain. If definite low water is indicated below the gauge-glass level, close the main steam valve and feedwater valve.
3. If the boiler is equipped with one, open the superheater drain.
4. Continue operating forced-draft and induced-draft fans until boiler cools gradually.
5. Let the pressure reduce gradually and when the furnace area is sufficiently cooled, check for leaking tubes and other signs of over-heating damage. On firetube boilers, look for cracked or wrapped tube sheets, broken and leaking stay bolts in the water legs. On scotch marine boilers, check for cracked or leaking furnace-to-tube sheet welds. On cast-iron boilers, look for cracked sections. On steel boilers, check for leaking joints on longitudinal or circumferential welds or riveted joints.
6. If no leakage is evident, give the boiler a hydrostatic test of 1-1/2 times the allowable working pressure. Then again check for leak-age at all critical parts of the boiler. If leakage is observed during the initial check or during the hydrostatic test, notify the authorized boiler inspector immediately so that she or he can inspect the boiler and advise on permissible repairs.

TESTING THE LOW WATER CUT-OFF

As mentioned in a previous post, one of the most important safety controls on a boiler is the Low Water Cut-off of which there are two on a boiler. Both the primary and the auxiliary LWCO need to be tested on a regular basis to keep your boiler operating in a safe manner.

Here is a procedure our service techs use to check Low water cut-off’s -

1. Blow down the water column and observe that the water level in the sight glass returns quickly.
2. Confirm that the shunt switches are working properly.
3.  Place the boiler control in manual and set the firing rate at a minimum.
4.  Observe the water level in the sight glass.
5.  Close the feedwater valve to the boiler in order to generate a slow drain condition.
6. Confirm that the feedwater pump re-circulation line is in place and operational, so that the pump will not dead head.
7. While the boiler is operating, observe the water in the sight glass noting where the first LWCO shuts the boiler off. Make sure the water level does not leave the sight glass.
8. Open the feedwater valve to the boiler in order to establish a normal operating condition.
9. Again, observe the water level in the sight glass.
10. Override the first LWCO, take note of the water level in the sight glass, and operate until the second LWCO shuts the boiler off. Remember; do not let the water level leave the sight glass. 
11. Open the feedwater valve to the boiler in order to establish a normal operating condition.
12. Reset the second LWCO.

We also replace the low water cut-off gaskets as part of our standard maintenance procedure. In fact we include these with many of our gasket kits that we provide for boilers.