Reciprocating / PD Pumps

The transient effects of the operation of reciprocating pumps requires careful consideration at the design stage as these pump types are capable of subjecting pipework to very high pressures and / or high levels of vibration. The pumps can effectively deliver the required system flowrates at very high pressures and consequently full capacity relief valves will need to be installed on the system if there is a risk of a valve closing against pumped flow. Alternatively, high pressure pump trip switches need to be installed at locations throughout the system to ensure the pumps cannot operate against a closed valve. These issues can only be accurately resolved through mathematical modelling and Hydraulic Analysis Limited have analysed many different types of reciprocating pumps, from 10,000psi Triplex Mud pumps to gear type pumps.

The studies are undertaken in accordance with API 674 and a full vibration analysis is normally carried out. The main driving force between these studies is to ensure that the pumps do not operate at the natural pipework frequency which can result in significant system damage being incurred. Typical problems are discussed below (although there are many different types of reciprocating pump and the examples shown below are not an exhaustive list):

Triplex type pumps: These studies often require the sizing, calibration and optimisation of pump discharge pulsation dampeners to minimize pipework vibration and loading. We accurately model the cyclic outflow hydrograph from the pump, based upon the three pistons operating at 120º opposition. We will also analyse the effects of different flowrates (piston liners are sometimes installed) and pump speeds. We require the pump speed, piston diameter, crank length and stroke length to model the pumps. These pump types are often used for temporary flushing or HP Mud supply to well drills.

Reciprocating type pumps: These pumps are often used for fuel supply and house 8 teeth gears to supply flow to turbines. Past experience has shown that, if sufficient bypass flow or back pressure is not available in the system then vibration problems can occur. If the system is not adequately modelled at the design stage then the pumps can operate at the natural harmonic frequency of the pipework. This is mainly due to the pulsation frequency which is generated by the pumps. A pump running at a synchronous speed of 50Hz will output flow at 200 Hz as there are 8 flow pulsations for every pump rotation due to the gearing mechanism.

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