REPORT ON A TEST OF A 350 ∅ HIGH PRESSURE PINCH VALVE
Introduction
Corflex Engineering (Pty) Ltd manufacture pinch valves, primarily for the mining and minerals processing industries. This test was at the manufacturer's request.
Standard Employed
Insofar as it is applicable to this type of valve, API 598 was adopted as the reference standard. In particular, the sections "high pressure closing test" and "maximum allowable leakage rates" (Table 6) were used as being the most appropriate indicators of successful valve operation and sealing.
Description of the Valve
A pinch valve is basically a rubber hose or sleeve which is clamped in a pipeline and pinched or squeezed together to stop or control flow, normally a fluid and often a slurry mixture of carrier fluid and solids.
The sleeve is contained and supported in a body frame, which frame carries and supports the closing mechanism, which is vice-like, having the pinchers acting on the centre line of the sleeve. See Figure 1.
The sleeve in a High Pressure valve is typically hand constructed from moulded rubber with steel cord reinforcement. It is generally lined internally with a layer of natural rubber to provide additional abrasion resistance.
The pinching mechanism can be manual or power-assisted (hydraulic, electro-mechanical, pneumatic, etc.) depending on the pressure of operation which in turn determines the closing force necessary to fully compress the sleeve and form the seal. In this sense, the valve could be considered a form of "resilient seated valve" as provided for in API 598.
It must be noted, however, that the sleeve, and therefore the fluid, is completely isolated from the body of the valve and the closing pinchers. There is no penetration of the sleeve by spindles, operators, etc. and as such, neither the construction nor the operation of this type of valve is fully envisaged by the specification.
The Test Rig
This is shown above and comprised the valve, the hydraulic power pack, and the pressurising hand pump. The fluid used was potable water.
The up-stream flange was fitted with an inlet hose at the 'top' of the sleeve where it meets the flange, and able to be pressurised via the hand pump. A pressure gauge and safety relief valve was fitted on the centreline of the flange.
The down-stream flange was similarly fitted with a pressure gauge and relief valve. The outlet in this case, also at the 'top' of the sleeve, was an up-stand of hp pipe so that any leakage (droplets of fluid) could be observed.
The Test Procedure
- The valve (open) was filled with potable water and pressurised to a nominal 8 bar to fully expel any residual air and to provide a back-pressure to the closing mechanism so that the sleeve could be pinched in as normal an operating manner as possible.
- The hydraulic pressure was applied and the valve was fully closed.
- The up-stream side was then pressurised to 22 bar – nominal operating pressure plus 10%.
- The down-stream side was exhausted to zero pressure via the stand-pipe.
- Leakage across the 'seat' was observed as a droplet of water exiting the stand-pipe.
Observations
Leakage was observed as a droplet of water forming on, and falling from, the stand-pipe, every 90 seconds. This was observed for a total of 15 minutes: the frequency of the droplet formation did not vary to any measurable extent.
In this period, the up-stream pressure did not reduce to any measurable extent.
The test was repeated and the same results observed.
Conclusion
The valve is not, and is not meant to be, gas tight, neither in design and construction or in operation. The insignificant amount of leakage observed indicates that it is, and will be, liquid tight to all reasonable standards of service, and as such, I can certify that it is suitable for the fluids and operating pressures recommended by the manufacturer.
Insofar as API Standard 598 applies, the valve complies with the intention and objectives of the Standard.
Further Observations
The test procedure is simple and not particularly expensive or time consuming. Given that each sleeve, while manufactured to tools, dies, and controlled specifications, is essentially hand built, it may be of utility, to both the user and the manufacturer, to test each valve for duties that are considered high, or extra-high pressure, or other severe operational conditions.