Tuesday, March 4, 2014
Hydrostatic pressure testing is a test to check strength and leakage in a pressure vessel or components such as tank , pipe, tube , boiler, divers tank (SCUBA tank , Self Contained Underwater Breathing Apparatus ) and others , using liquid as a medium for transferring pressure . Read about hydrostatic test in Wikipedia.
The liquid used is usually water or oil. Water is more frequently used because it is cheap and easy to obtain . Water also rarely has side effects such as damage to rubber , sealant , and other parts . If water can cause corrosion to component material, water can easily be drained and dryed right after testing . Sometimes colored liquid is used to make it easy for visual inspection in case of leakage .
Why use fluid to transfer pressure? Because fluid can only be compressed very slightly. So it does not require huge power to pump much fluid volume in order to increase the pressure .
If using air or gas , it would be an explosion if the component being tested cannot contain the pressure . This is due to the nature of gas that can be compressed, if component being tested cannot withstand the pressure then explosion will occur because gas efforts to release it’s pressure as that gas is compressed.
Please see the picture above. For example: air from the atmosphere is compressed up to 10 times smaller in volume so that its pressure rise 10 times atmospheric pressure. The left picture is normal gas with Y piston height. In the right picture is compressed gas with piston height equal to Y/10 to rise pressure 10 times. If the component being tested cannot withstand that pressure, compressed air will be tearing apart the component being tested, in other words explode. Because that compressed and shrunken air, will try to expand until it reaches the original volume. The compressed air will stop expanding when the pressure is back to as low as the atmospheric pressure.
Fluid is only slightly compressible. If component being tested cannot withstand the pressure, a small leak can reduce much pressure. So component being tested will not explode to release the pressure of compressed fluid in that component being tested.
In the left image above is normal fluid, with Y piston height. The right picture is compressed fluid with piston height 9/10 of the original height (Y), but the pressure is risen up to 10 times. Just like a gas, pressurized fluid also try to release it’s pressure. But if there is a little leakage, pressure will drop drastically, it does not have to explode to release the pressure. Therefore, a slight leak will be easily readable on pressure gauge, and then inspected which part is leaking.
If the pressure decreases when tested, this is a clue that component being tested is leaking as it cannot withstand the pressure. Then do visual inspection to find out which part of component being tested is leaking fluid.
Hydrostatic testing is usually done with the pressure reaches 150 % of maximum operating pressure of component being tested, depending on applicable safety regulation.
I need a hydrostatic test to ensure the strength of a transparent cylinder made of Unsaturated Polyester Resin (UPR). The resin cylinder will be used as a sight glass / tube.
Hydrostatic test is quite expensive. A hydrostatic hand pump price is about US$ 100 with a maximum pressure of 700 PSI or 48 bars. But I need pressure as high as 1500 PSI, approximately 50 times higher than family car tire pressure, 102.1 times higher than the atmospheric pressure at sea level..
Then I remembered that I still keep an used clutch slave cylinder of CJ7 Jeep, and then I got the idea to utilize that clutch slave cylinder.
The inside volume of clutch slave cylinder is about half of volume of the inside resin cylinder, so it can easily be concluded that the clutch cylinder can pump enough water to rise pressure.
Clutch cylinder rubber boot and cylinder rod are removed. Please see the picture below. At the top is clutch slave cylinder, and at the bottom is resin cylinder with pressure gauge.
A fabricated threaded double nipple is used to connect resin to tee joint. A pressure gauge is connected to that tee joint. Another fabricated threaded double nipple is used to connect the last remaining end of tee joint.
Plunger or piston in clutch slave cylinder is pressed by a 12 mm bolt with a length of 120 mm . There is a nut welded to a plate frame. The plate frame is tied to clutch cylinder body with two long bolts. We can see a long bolt at the top is hooked to the clutch cylinder body. While another long bolt at the bottom is tied with 2 clamps, because I do not want to weld that bolt to clutch cylinder body.
As the nut is welded on the frame which is attached to the clutch cylinder body, so if the 12 mm bolt is rotated to the right, that 12 mm bolt will move inward and press the plunger into the cylinder body. That is similar working principle to bearing puller or pulley puller. In this case the clutch cylinder is pulled, while the plunger is pressed into the clutch cylinder by 12 mm bolt.
The plunger will put pressure to water in the clutch cylinder. Because the clutch cylinder is connected to tee joint , and that joint tee has resin cylinder on one end, and also a pressure gauge on the last remaining end of that joint tee, then transferred pressure by water will press the resin and the amount of pressure can be read on the pressure gauge.
Resin cylinder has top and bottom steel caps which are secured by 4 long bolts, and sealed by Dextone red silicone sealant.
When testing with 1500 PSI pressure , it was proved that the resin cylinder is not cracked, and there was nothing water droplet come out of silicone sealant, also no water droplet out of the rubber seal of clutch cylinder. 1500 PSI pressure did not decrease within a few minutes , alias steady alias does not leak.
Please see the video below when testing is done.
If the video above does not open on this page, it can be viewed also in YouTube with title 'Very High Pressure Hydrostatic Test 1500 psi' with the following address: