Every organisation that uses pumps in its operation is always seeking to maintain pump performance and optimise their efficiency, reliability and minimise energy consumption. To achieve this, accurate measurements over a pump’s working life is essential. Thermodynamic pump monitoring is now accepted as providing the most accurate measurement of pump performance. Simon Cappleman – Pump Manager of Corroserve* outlines the principles of thermodynamic pump monitoring and highlights the benefits its use can bring.
The thermodynamic method of monitoring pumps is based on the first law of thermodynamics i.e. the increase in the internal energy of a thermodynamic system is equal to the amount of heat energy added to the system, minus the work done by the system on its surroundings. In basic terms the energy losses in the pumping process result in a temperature rise in the fluid.
The concept of centrifugal pumps is based on Bernoulli’s principle that there are three forms of energy in a fluid system – potential, kinetic and pressure. Kinetic energy and potential energy are the thermodynamic properties of the fluid system and the two energy quantities of heat and work are required to account for real world conditions.
The thermodynamic method for pump testing requires the following data: temperatures of the pumped fluid at pump suction and pump discharge; suction and discharge pressure; motor input power and motor efficiency. To measure the energy losses dissipated as heat, corrections are applied for the specific heat and density of the pumped fluid.
Corroserve’s test engineer uses the monitoring system to take readings from a fully open discharge valve until any fluctuations in temperature have settled out. The engineer will then continue to take readings for a pre-determined amount of time at set points of the discharge valve until the valve is fully closed. This will then map the pump curve from the fully open to fully closed position of the discharge valve. The pump curve will then be compared to the original test curve of the pump and any losses in performance or increase in absorbed power will show signs of wear.
As the running clearances open up between the wearing parts of the pump, the leak paths of the pumped fluid increase, resulting in lost efficiency. A thermodynamic monitoring system detects and records these losses more accurately than other test methods. During the test a filter can be adjusted to take readings from every 2 seconds up to every 3.3 minutes. This allows the system to achieve a steady reading of Delta T (temperature differential) ISO standards recommend a steady fluctuation of Delta T equal or less than 5 mK/min.
Corroserve uses the latest thermodynamic pump monitoring equipment to monitor the performance of any Class C industrial centrifugal pump over its long working life. The degree of accuracy of the test for centrifugal pumps is to ISO 5198 Part 111 (formerly ISO 2548). Due to the mathematical equations designed into the software, such as the Affinity law equation of flow, only centrifugal pumps can be tested by this method.
The latest thermodynamic pump monitoring equipment measures pressure, temperature and power input. This information is then used to calculate flow, which eliminates the need for a flow meter. As a result, tests can be carried out on equipment with only short lengths of straight pipe at the suction and discharge (2 x diameters). Conventional testing methods require at least 5 x diameter lengths of straight pipe.
A completed TPM Data sheet is provided to the pump user and this can be compared against the OEM test results when the pump was new. A report is also issued showing what efficiency gains can be expected following manipulation of the impeller and after the application of an energy saving protective coating; Fluiglide.
A vibration test and sound readings are also taken to check for rotor imbalance, bearing wear and cavitation. The end user can then make informed decisions about refurbishment and the optimum time for any work to be carried out. The financial investment in testing will be quickly recovered in savings made by reduced energy costs from running a more efficient pump.