How much leak is a leak when testing?
Bronkhorst has recently been involved with the manufacturers of valves and valve seat testing machines. We were able to assist with a change in measurement technique from pressure degradation to quantifying an amount of fluid passing the valve seat under test.
Valve manufacturers currently check any metal to metal valve seats using pressure degradation. The most common test is a protocol defined by the US (ANSI) valve leakage standard, for metal to metal sealed valves this is Class IV, one of 6 classifications. The test classification is dependent on the sealing method within the valve. Each of the individual classifications has a maximum allowable leakage; Class IV is 0.01% of rated capacity. There are also standard test pressures and procedures. The test pressure for Class IV is 3-4 barg or within +/- 5% of the maximum operating differential pressure, whichever is less. The test procedure is: Pressure applied to valve inlet with outlet open to atmosphere or connected to a low head loss measuring device, full normal closing thrust provided by actuator.
Data acquired on leakage and flow shall be accurate to +/- 10% of reading. Readings are taken by logging the pressure reduction over a time period, this is difficult to administer and is very labour intensive. The manufacturers rely on an operator to use a stopwatch and a pressure gauge, recording the pressure drop and comparing this with the specification extrapolated from the original valve design specification.
Manufacturers have been looking at this process and consulting with
Bronkhorst to support their method development. Rather than measuring pressure
degradation the manufacturers are now looking to measure the flow across
the valve seats under test. The two main reasons for this are:
1. Reduce testing time 2. Improve accuracy.
Added together these elements can realise substantial savings in testing time and better understanding of overall performance. Bronkhorst, following consultation with the valve manufacturers recommended the Bronkhorst mini CORI-FLOW ML120 instrument. This Coriolis mass flow meter gives a measure of very low flow leak rates with a very fast speed of response. This gives the operator an instant reading for a pass or a fail which means production time reduces and productivity increases. The accuracy of the mini CORI-FLOW instrument used in the testing process far exceeds the requirements of the protocol, however this means that finished product quality is improved. Integrating new technology in the system allows the manufacturers to access outputs for data logging the flow profiles and also the direct performance of each valve seat. This has given management a great tool to help with continuous improvement practises.
One manufacturer has taken the flexibility of the Bronkhorst mini CORI-FLOW ML120 a step further and by making use of the high pressure rating of the instrument is able to perform testing protocols that simulate real world high pressure applications. The results are more relative to the product performance and this in turn has offered a new level of confidence to their customers.
Technical specification: Instruments rated to 400 bar, accuracy of +/-0.2% of reading with liquid and +/-0.5% of reading with gas, certified for ATEX Zones 1 and 2, ability to be calibrated to ISO 17025, on-board PID control is standard, as is over-run protection and a self-learning function, with a turn-down ratio of 2000:1. Flow rates as low as 50 milligrams per hour have easily been achieved within a surprisingly compact physical unit (“the smallest Coriolis instrument in the world”). Multi-parameter instrument with flow, density & temperature outputs. The closed loop PID control board enables control of any pump technology and direct valve control, other types of control valve or indeed shut-off valves can be engineered depending upon the process conditions as required. Multibus communication is also possible, RS232/RS485, Modbus, Profibus, EtherCat, etc. The instruments can be assembled in a compact PLC controlled box offering a plug & play solution. Using Multibus communication master/slave control can be achieved.