Centrifugal pumps are the most widely used pump type in the oil and gas industry. They are the workhorse of refineries and petrochemical plants, with arguably millions of centrifugal pumps in operation worldwide. Operators rely heavily on their smooth and uninterrupted operation to meet daily production targets. Hence, it is absolutely necessary to accurately size a centrifugal pump for proper and reliable plant operations.
Following are some of the best practices that can be followed for properly sizing a centrifugal pump:
- Gather the essential Information: The basic information required for sizing a centrifugal pump is, but not limited to, the following documents: Process Design Basis, Project Design Basis, Process Flow Diagram (PFD), Piping & Instrumentation Diagram (P&ID), Material Balance tables, Preliminary Plot Plan, Equipment Datasheets, etc.
The above documents will provide different pieces of information that will be required for pump sizing calculations. For example, the Process/Project Design Basis document will provide information like over-design margins for specific equipment, turndown ratio, and battery limit conditions for product and utility lines, etc.
Process Flow diagrams will show the number and sequence of equipment in the pump circuit as well as the locations of control valves. P&IDs will show details of valves, fittings, instrumentations (FE’s) and equipment elevations, etc. Material balance tables will provide flowrates and physical properties like density, viscosity, vapor pressure, etc. of the service fluid for different operating scenarios and will help decide on the design case and rating case.
Equipment datasheets will provide allowable pressure drops through different equipment. A preliminary plot plan will provide distances and elevations of the various equipment involved in the pump circuit.
- Draw the pump circuit: A well-drawn and accurate sketch of the pump circuit is a good starting point in pump calcs. Such a sketch would show all the major equipment on the pump suction and discharge sides, and if applicable, all the different destinations where the service fluid needs to be pumped. For example, an overhead product pump of a distillation column may normally be pumping liquid back to the tower as reflux, and to product OSBL storage tanks, but will be in total recycle mode during start-up time. Hence the pump needs to be sized to operate under all scenarios
- Design case and rating case: Selecting the right design case is critical for proper pump sizing. Usually a centrifugal pump has multiple operating cases with different flowrates and operating conditions (T&P), thereby resulting in different fluid physical properties for each scenario. In some cases, the difference between the max and min operating flowrates can be huge. As an example, the max flow case can be 120% of the case with highest normal operating flowrate and min case can be 50% (turndown case) of the case with lowest normal operating flowrate. In such scenarios, the final pump design must be checked for lowest flowrate case to ensure the pump does not operate too high or low on the curve. Otherwise, multiple pump (2+1) operating options should be considered.
- Importance of pipe fittings: For a grass-roots project, information related to pipe lengths, number of valves & fittings and equipment elevation may not be available early on, these must be generously assumed to avoid under sizing the pump. It can be argued that the pump suction side requires extra careful assessment as it directly impacts the NPSHa calculations. A process engineer must consider any temporary or permanent strainer, isolation valves, tees, elbows, and reducers in the suction line pressure drop calcs. The discharge side should include any equipment, fittings or instruments that can contribute to pressure loss.
- Assumptions: All assumptions related to calculations must be made carefully and noted in the calculation file. These include number of fittings, valves, size of reducers, equipment and piperack elevations, equipment pressure drops, liquid levels, plot plan locations, etc. These must be revisited and verified along the way, for example after 3D model is developed by the piping group, or after receiving actual information from the pump vendor.
- Remember the phase change: In many pump circuits, the liquid being pumped is heated in a heat exchanger or a fired heater before reaching the final destination – which can be a distillation column or a reactor/pressure vessel. In such cases, it must be remembered that the pressure drop of a two-phase line in much higher than the pressure drop of a liquid-only line. Therefore, the extra pressure drop encountered by the pump after the phase change must be added to the pump discharge pressure calculations. The equipment layout on the plot plan can be ‘tweaked’ to minimize the length of the two-phase line while trying to keep the fluid in the liquid phase for as much of its circuit as possible. This greatly simplifies process design as well as subsequent plant operation.
- Casing design pressure: Some clients like to calculate the pump casing design pressure based on max. operating pressure in the suction vessel PLUS liquid static head at high Liquid level (HLL) in the suction vessel. While other clients prefer the worst case scenarios of pump suction vessel at PSV set pressure (relieving conditions) PLUS high liquid level (HLL) in the suction vessel. This is matter of project guidelines and must be discussed and agreed with the client beforehand.
The above mentioned are some of the few best practices that can be followed to ensure an accurate pump design. Of course this does not cover all scenarios and sound judgement must made on case-by-case basis.
Amit Kanda
GK Process Engineerig LLC