By Darla Jean Thompson, Marketing Manager, CAT PUMPS
Given proper maintenance, pumps should provide years of flawless performance. Unfortunately this is not always the case. Poor system design, improper maintenance techniques and misdiagnosis of system problems can lead to premature failure. While the cost of replacing a pump can be expensive, the cost from lost production resulting from an unscheduled outage is also significant. However, if a maintenance program is established, the vast majority of pump problems can be prevented.
Stainless steel pumps used in gas turbine fogging systems. Photo courtesy of CAT PUMPS.
It is very important that that the maintenance recommended by the pump manufacturer be carried out on a regular schedule. Although the maintenance will vary for different pump designs and applications, it generally includes the following:
- Daily: Clean the inlet supply line filters. Check the oil level and quality. Inspect for oil or water leaks.
- Weekly: Inspect and adjust the belts or pulleys. Inspect the plumbing for loose fittings, worn hoses or other problems and correct any problems immediately.
- At 300-500 hour intervals, change the oil. Specially formulated oils are generally changed every 500 hours and non-specialty oils every 300 hours.
- Inspect the seals and valves at 1,200-1,500 hours for wear and replace as needed. If there is no evidence of wear the seals and valves should be rechecked after each 500 hours of operation. The seal and valve life can vary according to the pump’s application, the liquid being pumped, the temperature of the liquid, and the bypass and inlet conditions.
To simplify maintenance the inspections/replacements can be scheduled to coincide with oil changes. However, should the operators observe a drop in system pressure, noise, vibration or leaks, maintenance should be carried out immediately. Although the schedule just outlined will meet the needs of most installations, it may be necessary to do some of the inspections more frequently in harsh operating conditions.
Before starting any pump maintenance it is important to determine what work needs to be done and if the pump has had any operating problems such as low pressure, leaks, noise or problems in starting. This information can be determined from the operator’s checklist/log. An operator’s log allows the maintenance staff to see changes in operation over a period of time. Logging of operational data also allows a plant to plan ahead and make adjustments to the maintenance schedule if needed.
Before performing any maintenance it is important that the staff prepare for the outage.
- Do they have the right tools for the job?
- Are there any special tools required?
- Do they have the correct service manual showing the current parts and numbers?
- Is there any specialized pre-training required? This could include reviewing a service video or manual before replacing seals, valves or other components.
- Do they have the correct torque information for the bolts?
- Are there any technical updates from the pump’s manufacturer? (This can be found by checking the pump manufacturer’s website.)
- Does the plant have service kits available or do they have to be ordered?
After all of the maintenance has been completed it is essential that the manufacturer’s recommended steps for bringing the pump on-line be followed. This includes:
- Filling the pump crankcase with the correct amount and type of oil.
- Hand rotating the pump to ensure that it is operating smoothly.
- Inspecting the drive belts for proper tensioning and/or couplings for secure fit and correct alignment.
- Checking all the plumbing connections.
- Making sure the inlet and outlet valves are open on the pump.
Once these have been completed the pump can be started and the primary pressure relief and secondary relief valves adjusted. The secondary relief valve should be set approximately 200 psi above the primary valve setting.
Darla Jean Thompson, marketing manager, has been with CAT PUMPS for 33 years and through her advertising and marketing efforts has promoted the company’s technical support with data sheets, service manuals, industry brochures and its website.
Pump operating problems
If a pump has worn seals and/or stuck valves the pump may run extremely rough. This would normally show up during preventive maintenance inspections. On the other hand, if the problems arise between inspections it could indicate that the frequency of the maintenance needs to be changed.
Foreign matter in the pump can cause the valves to stick. Although this can be easily rectified, the plant must determine the source of the foreign material or the problem will keep on occurring. Open supply tanks are generally the major source of foreign material enteringa pump.
If foreign material is not the problem the water line feeding the pump should be checked. Inlet restrictions or air entering the line can lead to severe cavitation and cause the pump to run rough or have low pressure. If one of these is the case, increase the size of the plumbing, remove any restricting fittings or elbows, and check for a tight seal on all valves and fittings.
Pulsations are usually caused by the absence of a pulsation dampener, a faulty pulsation dampener, or foreign material trapped in the inlet or discharge valves. The first thing that should be checked when this occurs is the pre-charge on the dampener. If the pressure is low the dampener should be recharged or replaced. If the pulsation continues, new valves may by be required.
Some pulsation is inherent in a triplex pump design and even more pulsation is present in a two-piston pump. When using a two-piston pump, the pressure varies from 60% above mean system pressure to 100% below mean. Although pulsation dampeners can help to even this out, it may not be enough. If not corrected, pulsation will cause damage to the pump, couplings, valves and other components. Incorrectly sized reservoir tanks can also cause operating problems. A reservoir tank should be 6-10 times the system’s capacity (Figure 1).
Premature seal failures are another concern with pumps. Seal failures can be caused by:
- Misalignment of the plungers
- Over pressure of the inlet manifold
- Abrasive material in the liquid being pumped
- Excessive pressure and/or temperature of the liquid being pumped
- Running the pump dry
- Water hammer
Fortunately, raising or lowering the inlet pressure and improving the filtration system can correct many of these problems.