Pump Gages Can Tell You More Than Meets the Eye
The Volunteers Corner
Pump suction and pressure gages can tell you more than what they are recording at any one moment. It’s your job to interpret their full meaning.
All gages should be read head-on at eye level for accuracy. You can get two different readings by looking at a gage from two different directions. Under fireground operating conditions, reading a pump gage to the nearest 5 psi is close enough for anything you wish to compute. The vacuum side of compound gages is calibrated in inches of mercury—usually with markings at 10-inch intervals with a maximum reading of 30 inches Hg (mercury).
The experienced pump operator frequently looks at the gages when the pumper is in quarters to determine whether the needles are at zero. And as he starts to hook up on the fireground, he takes a glance at the gages for the same reason. If the pressure gage reads 5 psi when the pump is motionless and no water is entering it, then this error must be compensated for in later readings. Sometimes the compound gage is off and the needle may erroneously indicate a few pounds of pressure or inches of mercury. Again, this indicated error should be taken into consideration.
Hydrant operation: After hydrant water has entered a pump but before any water flows out of the pump, read the suction (compound) gage to obtain the static pressure (the pressure of water that is motionless). As soon as the first hose line is charged and the nozzle is open, read the suction gage for the residual pressure (the pressure remaining in the hydrant).
The percentage of pressure drop from static to residual pressure gives you an idea of how much more water is available from the hydrant. If the drop is not more than 10 percent (for example, from 70 psi static to 63 psi residual) then the hydrant will probably be able to supply three more lines flowing the same gallons per minute as the first line. If the drop is no more than 15 percent, then the hydrant will probably be able to supply two more lines with the same flow rate as the first. If the drop is 25 percent, then the hydrant may or may not be able to supply a second line with the same gallons per minute as the first.
It is a good practice to maintain at least 10 psi on the suction gage of a pumper at a hydrant. If more water is a critical need, then the pressure may be allowed to drop to 5 psi, but only if the pump operator keeps an eagle eye on the gage to prevent the pump from drawing a vacuum. A pump should never be allowed to draw a vacuum from a hydrant because of the danger of damaging the hydrant and water main.
Danger signal: When the pump is taking water from a hydrant and the compound gage shows a drop in the residual pressure and eventually goes into a vacuum reading, it indicates that one of two things is happening. The pump may be demanding more water than the hydrant can supply and the condition known as cavitation is beginning to occur. Or the suction strainer is becoming clogged with scale or other foreign matter, which leads again to cavitation. In each case, the effect on the pump—and the suction gage—are the same. The difference between the two situations lies in the causes.
So it is up to the pump operator to determine the cause when he gets his first warning from the suction gage. The first situation, the pump demanding more water than the hydrant can supply, arises either when adding hose lines to the pump or when another pumper hooks up to another hydrant and robs water headed for the first hydrant.
The second condition, when scale or other debris clogs a suction strainer, is more apt to develop gradually, and this is reflected by a gradual drop in the residual pressure. Breaking down the suction line and cleaning the strainer is the only way of correcting this condition.
Keep an ear open: Confirmation of the fact that the pump is not getting enough water is indicated by the tachometer—and your ear. As you try to increase the pump pressure by increasing the speed of the motor, the tach shows more rpm but the pressure gage fails to show any higher psi. During any pumping operation, you should dedicate at least one ear to the sound of the motor. Thus you will be aware of the motor racing as you increase the throttle without obtaining a customary increase in pump pressure.
When drafting, the compound gage needle will be on the vacuum arc, informing you about the condition of the drafting operation. When the pump is primed and during actual pumping, the vacuum reading will indicate the height of the lift (the vertical distance from the water surface to the center of the pump). One inch of mercury is equal to 1.13 feet of water column. For practical use, you can regard an inch of mercury as equal to 1 foot of water column. Thus, if the lift is 12 feet, the vacuum gage should show a reading of about 12 inches of mercury. We say “about” because you can’t read the usual pump compound gage to an accuracy of better than 2 or 3 inches of mercury.
Next month, we will continue this discussion.