This article from Valutech Inc. provides an overview of the maintenance guidelines and safe operating procedures of positive displacement pumps, using Alfa Laval’s SRU line of rotary lobe pumps as a basis. Following proper maintenance procedures and respecting prescribed operating conditions and service intervals leads to more efficient operation and longer equipment life span for any piece of equipment or machinery, and positive displacement pumps are no exception. This style of pumps is suitable for specialty applications including the handling of high viscosity fluids at potentially high pressures, all while being able to cope with gentle handling of sensitive materials at lower flow rates — this leads to unique challenges and operating conditions to consider when discussing maintenance practices of positive displacement pumps operating in hygienic manufacturing industries, such as the cosmetics, pharmaceutical, biotechnology, food, beverage, and personal care industries, as well as other industrial and chemical processing applications.
Daily Operation Considerations
Positive displacement pumps must have their seal cavities vented and purged prior to start-up to avoid dry-running —failure to do so may lead to warping or cracking of seals, overheating of pump components, and general damage to rotors and casings that undergo metal-to-metal contact. Pump units should never be run dry unless a flushed mechanical seal is fitted.
All control and monitoring equipment related to the pump unit must be functional and accurate, including proper function of any signals and alarms related to thermocouples and pressure/temperature sensors. It is advisable to install pressure gauges on both the suction and discharge sides of the pump in order to correctly monitor and isolate any issues within the pump and/or system piping.
Some positive displacement pump rotorcase covers have the option to be fitted with heating/cooling jackets in order to heat the pumphead and maintain desired pumped media temperatures; each jacket will have its own maximum allowable pressure and temperature.
Pumps fitted with single or double flushed mechanical seals require flush media to operate correctly — this media, also known as the ‘barrier’ fluid, acts as a lubricant and coolant and should be supplied to the seal face at the required volumetric flow rate to dissipate build-up of heat at the seal face. Proper operating conditions include maintaining a pressure high enough to keep seals clean and prevent contamination from other media in the pump. In the case of flushed seals, the flush media must be at a notably greater pressure than the discharge pressure (e.g. 0.5 bar higher for single flushed or 1.0 bar higher for double flushed mechanical seals). In practice, end users should monitor the flow rate of the flush media (ex. greater than 30 L/h), and the temperature of either the flush media or the seal faces. In the case of single mechanical seal arrangements, the pumped media provides the interface film for the seal face — in other words, the end user must rely on the flow rate and temperature of the process media to ensure the process media itself provides sufficient cooling and lubrication.
Safety Checks
When starting the pump unit, perform a visual inspection prior to operation to ensure there are no abnormalities including: leaks, vibrations, loose components, unusual noises, etc.
A seal fitted with a thermocouple connected to appropriate instrumentation allows the user to have alarms in place in the case that the seal face temperature exceeds specified limits; a pump may have up to two thermocouples (one per seal). Thermocouples must be installed and maintained to appropriate standards for safe circuitry, and fitted to the correct shaft seal specified by the manufacturer. Thermocouples must be calibrated (e.g. within ± 5°C of pumped media) prior to start-up to ensure accurate temperature readings. To manually inspect the thermocouple, visually inspect that the thermocouple lead is fixed correctly without signs of damage (due to vibration, etc.), then produce a local temperature change at the stationary seal (e.g. by spraying cold fluid) on the back) to observe accurate changes in temperature readings.
Other safety checks before and during operation include verifying: correct oil levels and absence of contamination, signs of overheating, and ensuring operating conditions stay within the manufacturer’s specified limits.
General Maintenance Guidelines
To drain the pump head for maintenance procedures, remove the rotorcase cover.
The gearbox of a positive displacement pump requires the oil level to be maintained at the manufacturer’s specified level; typically the manufacturer will also recommend the type of oil, such as: BP Enersyn SG 150, Castrol Alphasyn, Mobil Glygoyle 22, etc.
The bearings of the gearbox must be changed at regular intervals — typically these intervals range from 2,000 to 30,000 pump running hours and are a function of the pump model, operating time, and the differential pressure of process media.
The following is the general procedure for replacing seals in positive displacement pumps, using a pump with a flushed seal as a basis, including the removal and fitting process:
- Stop flow of flush media and disconnect the thermocouple lead, ensuring it is clamped to the rotorcase.
- Remove the rotorcase, rotors, and cover.
- Carefully extract the stationary and rotary seal assemblies, O-rings, and any gasket or abutment spacer, taking care not to damage sensitive parts like the thermocouple wire.
- Lightly grease new O-rings for rotary and stationary seals to ensure smooth fitting.
- Place stationary seals into the rotorcase, ensuring proper orientation and thermocouple positioning.
- Slide the rotary seal assembly onto the shaft until aligned with the marked setting length.
- Secure rotary seals with grub screws, torque as specified, and tighten bush locking screws, ensuring the thermocouple lead is properly mounted.
- Reassemble the pump components, connect thermocouple leads, and verify proper operation and seal function at startup.
While these instructions apply to generally all PD pumps, it is always advised to consult the manual of the specific unit for any unique considerations and specific procedures. Correct mechanical seal function is dependant upon lubrication and cooling of the seal face; it is therefore imperative to avoid dry-running which can lead to the invalidation of both of these requirements. Note that it is normal for mechanical seals to undergo small, controlled amounts of leakage during standard operation. However, maintenance must be performed when this standard level is exceeded; generally, mechanical seals should not be removed unless they are to be replaced.
Spare Parts
While pump manufacturers will always recommend using their own spare parts for their products, often for good reason, the best general practice is to consult with the manufacturer, distributor, or other expert to ensure that the correct spare parts are used for the pump unit. For example, incorrectly specified seals may lead to improper sealing with mismatched cross-sectional profiles, or the use of seals that are not rated for the operating temperatures of the pump. These situations can provoke fluid leakage, product contamination, and other subsequent issues that lead to hazardous operating conditions and costly downtime.
In general, be sure to consult the manual for the specific product you are operating and follow the manufacturer’s guidelines for operation and maintenance. While this article provides a general overview of best practices, each specific product line will have its own guidelines, which can vary from application to application.