A reliable system for fuel supply to emergency generators is a common theme across NFPA requirements. My earlier articles on “Key Components of a Mission-Critical Facility” have covered automatic transfer switches and test load banks. This article will focus on the fuel transfer pump-set, another critical element of a reliable on-site power generation system.
There are several topics that should be considered before selecting and specifying a fuel transfer pump. The basic points are sufficient flow rate, electrical power requirements and suitable environmental protection. But what about system pressure regulation, control requirements, alarm monitoring and failure detection? Here is a step-by-step guide on how to properly select and specify a fuel transfer pump for a mission-critical application.
Flow rate: I normally recommend that a fuel transfer pump (for fuel supply to a day tank) be sized to have a flow rate capacity equal to 3x the maximum expected fuel consumption. This calculation should lead you to a pump that is, under worst case scenario, sized for a 20-minute “On” cycle, and a 40-minute “Rest” cycle. This ratio provides quick-enough fuel replenishment for depleted tanks, and allows sufficient rest time to conserve the pump’s service life.
Total Dynamic Head: Now that you have established the required flow rate, you will need to calculate the total dynamic head (TDH) for your installation. With the flow rate and TDH on hand, you should now be able to rely on the fuel pump-set manufacturer/vendor to select the appropriate pump and motor combination (if you would like to make your own selection, see this FAQ).
Electrical requirements: The size of the electrical circuits will be defined by the size of the electric motor(s) selected to provide the required flow rate. It is common practice to provide single-phase 115Vac electrical power to fractional horsepower motors (<1hp), and 220Vac power (single-phase or three-phase) to larger motors. Three-phase 460Vac motors are also common in larger (>1hp) applications. The power feed to the pump-set should come from a branch that is connected to the emergency power source. For mission-critical systems, I recommend that redundant power feeds be specified (should one power feed go down at any time, a “best source” selector in the pump-set’s control logic will automatically switch to the alternate source).
Environmental protection: a suitable enclosure is critical to the reliability and life expectancy of any equipment. A NEMA Type 3R enclosure should be the absolute minimum for fuel transfer pumps installed outdoors. In corrosive environments, a stainless steel enclosure should be specified to provide additional protection. Since pump set enclosures must be vented, an extremely corrosive environment might merit the use of stainless steel for all interior piping and valves. I would also like to point out that a corrosive environment is not limited to coastal installations. Water treatment plants or chemical plants can also be a very corrosive environment for outdoor steel structures. Finally, don’t forget to specify space heaters for pump sets installed outdoors in colder climates (<32°F).
Moving on to the finer details…
Redundancy: whether to specify a duplex or triplex pump set depends on the critical nature of the owner’s facility, the cost impact for unplanned outages, and whether the facility is staffed with maintenance personnel that can respond quickly to a pump failure. I recommend a duplex pump arrangement for facilities such as hospitals, emergency operations centers and data centers. In some instances, it is advisable to install redundant duplex pump sets, in separate enclosures and with separate power supplies. A typical duplex pump set will operate in a lead/lag mode, where one pump is selected to run, and the “lag” pump is only called to run if the flow rate of the “lead” pump fails to satisfy the fuel demand. The lead and lag pumps alternate at each call for fuel, therefore providing some balance to the total run-time for each pump. A triplex pump set behaves similarly, but with a third pump to add another level of redundancy.
Pressure relief manifold routing: A typical fuel transfer pump set will include a positive-displacement (P-D) pump coupled to a constant-speed electric motor. This design will produce a constant output volume of fuel for every rotation of the pump’s vanes. With a constant flow rate, these P-D pumps will build pressure in the fuel distribution lines depending on the restriction caused by friction losses, valves, elbows, etc. To avoid excess pressures, a positive-displacement fuel transfer pump should be equipped with a pressure relief manifold that discharges excess pressure back to the source fuel tank (the pressure relief manifold should never be plumbed back into the fuel pump’s inlet). Here are a few points worth mentioning in regards to pressure relief devices:
- The pressure relief device (whether internal to the pump or an external valve) is an emergency pressure relief device, and should not be used to “regulate” the fuel pressure in a closed-loop system where a manifold is continuously pressurized. When needed, a back-pressure regulating valve should be used to regulate system pressure (see below).
- An external pressure relief valve should be set to a level above the normal operating pressure. If not specified, this value is typically set at a default 65psi.
- Thermal expansion is a common problem that causes nuisance leaks in fuel piping systems. This article discusses this topic in more detail. Be sure to employ the use of thermal expansion pressure relief devices where fuel might be trapped in specific sections of piping.
- Back-pressure regulating valves are used to maintain a constant pressure in a piping system. These valves, once set to the desired system pressure, will gradually open and close to “regulate” the pressure in the piping system. The outlet of this valve is connected to a fuel return line that connects back to the main fuel source (tank). You should consider a back-pressure regulating valve (see example here) whenever there is potential for sudden pressure fluctuations in the piping system.
Control/Alarm parameters: The ability to locally monitor the pump set’s performance should be included in any technical specifications for a fuel transfer system. Basic features include manual and automatic operation modes, duplex lead/lag pump control logic, and monitoring and indication of normal and abnormal conditions. The owner may also want some of these parameters to be communicated to a remote control station or SCADA system. The designer should specify the preferred protocol (Modbus TCP, Modbus Ethernet, Fiber, etc), and the desired parameters to be communicated. Here are some of my recommendations:
- “System Not in Automatic Mode”: a system’s failure to start is often the result of someone simply leaving the control panel in the “Off” mode after performing maintenance. A fault or alarm should be initiated whenever the system is not in its normal “Automatic” mode.
- “Pump Running” indication: this is good basic information for the owner, and sometimes can be helpful in anticipating an alarm condition (i.e.: pump is running for excessive amounts of time, quick on-off cycling, etc).
- “No flow” indication: a flow switch installed on the outlet of each pump can alert the operator to a failed pump or motor, and a “no flow” signal can be used to disable a failed pump and to switch to a backup (lag) pump.
- A “Filter Clogged” indication is used to monitor a fuel filter/strainer that is installed upstream of a pump’s intake port. A pressure differential switch, installed across the filter, can alert the owner to needed maintenance before the pump fails by the inability to draw fuel through a clogged filter.
- “Leak Detection” indication is a “must have” for any pump-set equipped with a leak containment basin (and every pump-set should have a leak containment basin!). Should there be a failure in the pump-set’s internal piping, a leak sensor installed in the containment basin will stop the pump, and alert the owner.
- “Valve malfunction” indication: In some instances, the pump-set controller is responsible for “selecting” a fuel tank (one of multiple tanks) to be the source of fuel. This is accomplished by controlling electrically-operated valves to open or close on the fuel supply and fuel return manifolds. Should there be a valve failure to open or close, it is important that the system notify the owner immediately. I recommend that motorized ball valves be used in place of solenoid valves, as they include limit switches that indicate the valve position at all times. Using these limit switches, a pump-set controller can monitor the successful travel of a valve to the intended position.
There are a number of other finer details that will surely come up in more sophisticated installations, but these points should give you a sense of some of the important items to consider when specifying a fuel transfer system. When you have unique circumstances and requirements that are not covered here, I encourage you to reach out to me, or your preferred equipment vendor, to address your specific project one-on-one.
Fuel systems can be a specialized design area, but working with experts in the field, you can develop a safe and reliable fuel system to keep your client’s facility operating when it is needed most. I hope you found this article to be helpful, and please feel free to post your questions or comments. I appreciate your feedback!