“The Problem with Process Pumps”
We have many pumps in our everyday lives that give good service with practically no maintenance. The standard automobile has 7 or 8 pumps on it that operate 18-20 years with zero or minimal maintenance.
When did you last change the bearings on the radiator water pump of your car? When did you last change the mechanical seal on this pump.
A rotary air conditioning compressor is a pump adapted to handle air or gas. When did you last change the mechanical seal on the AC compressor in your house or car? The shaft seal on your AC compressor runs for many years without leaks. Wouldn’t it be nice if it were that way in industry?
Too many industrial pumps go through mechanical seals and bearings, like a newborn baby goes through nappies. Maintenance and reliability engineers constantly struggle with their pump seals and bearings. So, what’s the problem with process pumps?
It turns out that the pumps in your home and car are all perfectly mated into their pipe systems. The energy designed into the pump meets the energy requirements of the pipe system.
Too many industrial pumps are poorly mated to their systems. The energies don’t match. And, that’s the problem with industrial pumps.
The system is the pipe system, composed of tanks, collecting headers, pipes, elbows, filters, valves and other process devices (probes, heat exchangers, restrictor plates, etc.) Transporting a liquid through a pipe system requires energy. The pump has to meet or overcome this energy.
This energy has up to four elements:
The elevation differential,
The pressure differential,
The velocity energy, and
The friction energy (losses) in the system.
See the attached pictures of pump curves.
Notice the single black triangle drawn onto the curves by the design engineer. That single triangle on the page indicates the pump will perform at these precise head and flow coordinates. This means the design engineer considers the system energy is static, and the pump’s duty coordinates won’t stray.
I work as a pump consultant. I talk frequently with plant engineers who tell me, “The pumps don’t need gauges because the design engineer says the pump will operate at the head and flow coordinates indicated by the triangle”. And, they believe it.
This is a problem because the system is DYNAMIC. Levels change in the tanks and vessels. Pressures change in headers and reactors. Resistance changes with actuating valves and clogging filters in the system. The process pump migrates constantly on its performance curve as the system energies change. Useful energy is converted into vibrations, heat, noise and component distortion as the pump migrates on its curve.
If the design engineer thought the system were dynamic, he’d draw more than one triangle onto the pump curve, indicating more than one set of duty coordinates. RIGHT?
The misunderstanding generated by that single triangle confounds the reliability engineer and stalls the MTBF statistics on rotating equipment. That triangle gives the vibration techs something to ‘trend’.
Let me offer a revelation never expressed before in any technical book or pump magazine. Remember where you read it first. “The variable system energies are the continuing, undetermined vibrations viewed every day on the vibration technician’s accelerometer.” I will explain.
Certain vibration readings are normal and expected when the vibration probe touches running equipment. For example, a 4-pole electric motor in South Africa (50-Hz electricity) will produce a vibration signature at 25-Hz (1,500-rpm) and at 100-Hz (motor hum) with harmonics. The vibration technician expects to see this when the electric motor is running.
Other vibration readings are not normal and require correction. A pump suffering cavitation will leave recognizable vibrations between 3,000 and 5,000 Hz as the vapour bubbles collapse. A running centrifugal pump with a bent shaft will produce a vibration bump with every rpm on the vibration meter. Vibration technicians receive training to interpret other anomalies like imbalance, structural resonances and harmonics.
If the vibrations indicate misalignment between the pump shaft and the motor shaft, the engineer can order to stop the pump at some convenient moment and go through an alignment procedure without disassembling the pump. Once aligned, the vibrations from misalignment will calm down.
Maybe the vibrations indicate the pump is suffering cavitation. Most cavitation can be corrected without stopping the pump. To reduce or eliminate most cavitation, the operator or engineer must either:
(a) Increase the energy arriving into the pump, or
(b) Adjust the pump