The air conditioner failed to cool my house on a summer day earlier this year. I called Dean’s Air Conditioning where Ron Dean asked, “What’s the problem?”
I told him that the AC wasn’t cooling my house, which is about the limit of my knowledge of home central air conditioning systems. After Ron check out my AC system, I found out that the compressor was bad. This was a problem because a new compressor might stretch my budget. I considered myself lucky if the unit was just low on refrigerant gas.
Within 5 minutes, Ron pronounced that an electrical relay had shorted, and failed to trigger the compressor on the prompt from the thermostat. To my relief, I only needed to pay for a $7 relay and a service charge for the visit. My home AC unit has performed without problem since that day.
I’ll definitely call Dean’s Air Conditioning the next time I have a problem.
My neighbor went to the doctor complaining of a backache. He thought he had strained a muscle while cleaning leaves from the gutters around his house.
Read more about the Pump Guy’s Affinity Laws here. |
My home AC unit and my neighbor’s kidney illustrate how professionals have specific knowledge and are able to draw on information that the casual observer might not know. Often, these tidbits of information don’t appear in a textbook either. And this brings me to today’s Pump Guy column.
If you read the Pump Guy, you know I’m a retired maintenance practitioner. I’ve worked in refineries and steel mills, concentrating on pumps. I started my career as an apprentice mechanic and I completed my education 14 years later. I ended my employment days as a maintenance engineer and these days, I work as a pump consultant and offer lectures on pumps.
After viewing thousands of pump curves, I’ve noticed some striking similarities in pump performance that I made into three general rules. I call them “Larry’s Unwritten Rules,” and they’ve come in handy when I’m trying to work on a misbehaving pump and the engineer can’t find the pump curve. Now I’m sharing them with you.
Most industrial pumps carry a nameplate from the pump manufacturer. The nameplate normally shows the pump manufacturer, the model and serial number, and the pump size.
The pump size is normally listed with three numbers, such as “1 x 1.5 x 6,” “2 x 3 x 8,” “4 x 6 x 13” or “6 x 8 x 18.” A typical metric pump might be labeled “100 x 150 x 300 mm” (Figure 1).
The first number of the triplet is normally the discharge nozzle diameter, followed by the suction nozzle diameter, followed by the maximum impeller diameter for the housing. Some pump companies might switch or alter the order of the numbers. This doesn’t mean they have switched the nozzles (Figure 2).
Still other companies list the three numbers, and then indicate the actual diameter of the original impeller installed at the factory (Figure 3).
Of course, another impeller might actually be installed or the tag might be absent or damaged. If possible, recover this information while the pump is in the shop. If you know the impeller’s diameter, and the motor speed in rpm, you have a view into the pump’s ability to generate head/pressure.
Larry’s Unwritten Rules of Centrifugal Pumps
The First Unwritten Rule
With Imperial units at 1,800 rpm, the impeller diameter in inches, multiplied by itself or squared, is the approximate ‘Shut-off Head’ of the pump in feet.
