To increase the reliability of lube oil systems, it is advisable to use centrifugal single stage pumps whenever possible. This eliminates the need for relief and backpressure (bypass) control valves.
Malfunction of relief valves and/or backpressure control can cause an unscheduled shutdown of unit and hence result in significant revenue loss.
Single-stage centrifugal pumps can be used whenever the ambient temperature along with the use of thermostatically controlled reservoir heaters maintain an oil viscosity that allows the use of a centrifugal pump (oil viscosity is low enough to minimize the effect of viscosity on centrifugal pump power – low viscosity correction factors).
Types of pumps
Auxiliary systems that contain liquids use positive displacement or centrifugal pumps depending on the application. The screw pump and gear pump are used in systems containing oil. The centrifugal pump is used primarily for non-viscous duty, but can be used for oil systems if properly sized, and the efficiency and horsepower penalties are acceptable.
Regardless of the type of pump used, the function of all pumps in auxiliary system service is ‘to continuously supply the system fluid at the required pressure and flow rate’.
‘To continuously supply’ means that the pump must be capable of uninterrupted operation for the same period of time as the critical equipment it is servicing. Critical equipment is designed for a minimum of three years’ operation between scheduled shutdowns. In order to attain this reliability, the pump and its unit components (coupling and driver) must be properly specified, selected and designed.
As a first step, the equipment must be properly specified. The American Petroleum Institute specifications (API-614) provide a good basis for specifying highly reliable pumps and stream turbine drivers. All equipment requirements and site data should be entered on a data sheet to ensure a correct selection of the pump. Again, each major component of the system must be treated the same as the critical equipment to ensure maximum system reliability.
Pump mechanical seals
In order to ensure reliable, trouble free operation, pump mechanical seals are recommended instead of shaft packing. A properly selected and installed pump mechanical seal in auxiliary system service can operate continuously for a three year period.
All specified pump operating conditions should be confirmed so that they reflect the actual system requirements. This step ensures that the second part of the pump function definition will be met: ‘to supply the system fluid at the required pressure and flow rate’.
There are two major classifications of pumps, and either can be used for auxiliary system duty:
- Positive displacement pumps
- Dynamic pumps
A positive displacement pump is a variable energy (head) device as opposed to a dynamic pump which is a fixed energy (head) device. As the specific gravity of a liquid (the ratio of the weight of a given volume of liquid to an equal volume of water at standard conditions) decreases, the energy (head) required to produce the same differential pressure increases proportionally.
A positive displacement pump can meet the required energy increase at essentially the same flow, whereas a dynamic pump cannot. The only way for a dynamic pump to produce additional energy is to lower the flow rate (assuming the pump operates at a constant speed).
Therefore, increased system energy (head) requirements will force a centrifugal pump to a lower flow, or decreased system energy (head) requirements will cause it to deliver additional flow. In the case of bearing wear, a larger ‘equivalent orifice’ would require the pump’s discharge system resistance, and result in increased flow rate.
On the other hand, a positive displacement pump will essentially deliver the same flow in the above case. But both classifications of pumps present problems in terms of meeting auxiliary system objectives that will have to be solved using a reliable control scheme.
Viscous and non-viscous fluids
Auxiliary system pump applications involve both viscous (lubricating and sealing oils, etc.) and non-viscous fluids (water). The viscosity of a fluid is its tendency to resist a shearing force. It can be thought of as the internal friction that results when one layer of fluid is made to move in relation to another layer.
Positive displacement pumps are usually used for auxiliary systems containing oil, while centrifugal pumps are always used for large auxiliary systems containing non-viscous (water-based) liquids.
When determining if a centrifugal or positive displacement pump should be used in an auxiliary system containing oil, the following factors should be considered:
- Compare pump types of equal reliability
- Compare yearly operating costs based on normal (operating) liquid viscosity case.
- Compare driver and coupling size and costs for maximum liquid viscosity case.
- Determine critical equipment minimum and maximum flow requirements and compare control system complexity and cost.
The most common cause of oil system induced unit trips is the failure of the backpressure control valve to respond to transient system changes (trip or slow speed reduction of the main turbine driven oil pump). The use of centrifugal pumps eliminates the need for a backpressure control valve.
This best practice has been used since the mid-1980s to optimize the reliability of oil systems, and to achieve compressor train reliabilities exceeding 99.7 percent.