The pressure test is a key event in the installation of all Fusible PVC™ products, including Fusible C-900®, Fusible C-905®, and FPVC®. There are, in a lot of installations, more events beyond the pressure test that define completion of the project. For pipe supply and fusion services however, the pressure test can define completion of services in the manner expected.
In the majority of waterworks projects to date, the pipeline owner has determined the test pressure and duration of the pressure test for a fusible installation. The range of pressures has been from operating pressure of the system to 150% of the rated pressure of the pipe. The durations have been from 30 minutes to 24 hours. The normal average parameters have been 150% of the operating pressure of the system for a duration of 1 to 2 hours.
The primary standard used in pressure testing of PVC pressure water mains is the AWWA C605 "Underground Installation of Polyvinyl Chloride (PVC) Pressure Pipe and Fittings for Water". This standard states that the hydrostatic pressure test shall be performed at no less than 125% of the maximum anticipated sustained working pressure of the pipe, for a duration of 2 hours, unless this pressure exceeds the design pressure of the pipe or any of the appurtenances on the pipeline while performing the test.
AWWA C605 also contains the description of a "Test Allowance". This allowance is used for a typical gasketed pipe installation, and is defined as the "quantity of water that must be supplied to the pipe section being tested to maintain a pressure within 5 psi (34 kPa) of the specified hydrostatic test pressure". Installations requiring more water than what is permitted by the test allowance will not be accepted. It should be noted here that a pipeline that is predominantly comprised of Fusible PVC™, joined with butt-fusion joints and installed per the manufacturer's instructions, will not leak at the joints. This information regarding "Test Allowance" is for reference and those sections that contain gasketed fittings and connections or areas of other pipe technology. The following table provides the hydrostatic test makeup water allowance per 1000 feet:
Figure 2.1 - AWWA C605 Table 2
2.1 Preparations for the Hydrostatic Pressure Test
The pressure test is done after installation of the Fusible PVC pipeline. The line is installed, and in the case of open cut, backfilled except for any mechanical connections made to the pipe. In the cases of horizontal directional drilling, pipe bursting, and slipline applications, the pipe is completely installed and the ends where connections are made or are to be made left exposed. Also, for any of the above that require service connections, taps, blow-off valves, air bleeds, etc. these should be installed to the extent possible before the test. The determination of what to test is based on the rating of the in-line or attached fittings or devices. Each component must be reviewed to determine if it can handle the prescribed test pressure. Normally those that can't handle 125% of the operating pressure are removed or isolated from the test. In situations where this can't be done, the test pressure is lowered. For example, a pressure relief valve by its design is set to typically release at a pressure slightly above operating pressure.
The test is run from the lowest accessible elevation point in the test section. This is because the pressure in the pipe is made up of two components. First there is the line pressure (dynamic pressure) generated by the attached test pump. Second is the static pressure generated from changes in elevation. With water, for every 2.3' (+/-) that the line elevation is higher than the test location, an additional 1 psi (+/-) of head pressure is added to the line pressure as measured at the test point. So if the test pressure is 150 psi and the pipe being tested has an elevation increase of 23 feet, the high point will see a pressure of 140 psi.
Another reason for testing from the lowest point is to gain some advantage in purging the entrapped air in the pipe. Air in the line during the pressure test is problematic and a safety concern. Air is very compressible. As it compresses, it stores energy, that when released can create a serious safety hazard. This does not pertain to the PVC pipe as much as it does to the end caps, restraints, and testing hardware. It is far more likely for testing hardware to become compromised during a test, and create a safety hazard. With an air pocket behind it, the end cap or other testing hardware can become a projectile. Air changes in volume with changes in temperature. It takes considerably longer to reach test pressure by compressing air than it does water.
Air is removed from the test section by venting and by flushing. In the case of venting, this may involve tapping into the PVC line to relieve air at high points. This could mean an excavation and tap saddle/sleeve operation. . In most systems, air relief stations are included in the design for larger diameter transmission lines. Flushing is moving water through the line at high velocity, usually more than 3 ft/sec. This is more cost effective, but requires a disposal method for the water flushed if not done while the system is connected. Water is run at high velocity to move air along, and then the flow is stopped to allow the air trapped in the water during flow to migrate to the next high point in the system. This is repeated multiple times to remove air. Generally, moving approximately three volumes of water through the pipeline, with one volume being equal to the amount of water in the pipeline when full, provides sufficient flushing time. The recommendations of the current AWWA C605 and AWWA M23 should be followed for all flushing procedures.
The following figure 3.1 is an example of flow rates and tap sizes needed to produce sufficient velocity to flush a water main:
Figure 3.1 - Recommended Flushing Parameters
Normally testing is done with the line isolated from the balance of the system. To do this, end caps with taps for filling/draining water, measuring pressure, and connecting the test pump must be installed with the appropriate restraints. These are usually restraining glands designed for PVC or an equal type of PVC restraint. These parts must be procured with the end test pressure in mind. PVC restraints have a maximum pressure rating associated with them. There are special restraints that can increase the restraint pressure rating. In most cases, the end test hardware determines the maximum test pressure and not the pipe. All components of the system should be installed, checked, cured, and otherwise verified that they are capable of handling the test pressures associated with the test. This includes proper curing of any thrust or kick blocks installed on the pipeline.
The pressure test should be done with clean potable water in the case of water mains. Do not test with non-potable water. This could contaminate the system making disinfection harder and more costly.
2.2 Determination of Test Pressure
Test pressure is normally based upon the working pressure of the piping system. Working pressure is the long term pressure at which the system is expected to operate. This is nearly always different, and less than the pressure class or pressure rating of the pipe.
2.3 Steps of the Hydrostatic Pressure Test
The following steps are typically followed in a hydrostatic pressure test on a water main:
The end caps are usually tapped in the center. If venting is done from this location, it is highly likely that air will be trapped above the tap location. As a result, UGSI recommends installing screwed pipe fittings on the inside of the tap that allow an elbow and a pipe nipple pointing up toward the top of the end cap. This should come within 1/4" of the inside diameter of the PVC pipe being tested. Because the vent piping is on the inside of the end cap, it sees little to no pressure and therefore does not need to be watertight. Depending on the line configuration, the air vent can be reduced in size. If the line also will be flushed to remove air, then the vent line needs to be sized to handle the flow required for flushing. The end cap may also be tapped off-center and placed so that this off-center tap is at the top of the pipe cross-section, allowing a vent to be placed there that will remove the air that would normally be trapped at this location with a center-only type ductile iron cap arrangement.
If there are many changes in vertical direction, normally air vents are designed into the system. Where possible, these should be installed prior to a pressure test to facilitate venting of the line. Temporary taps and corporation valves or air relief valves can also serve to vent air pockets encountered during a pressure test situation, or they may be placed on the top of the pipe immediately before the end cap, to vent the trapped air above a center-only tapped DI end cap.
It is normal to test the integrity of a gravity sewer installation to confirm there is little or no infiltration or exfiltration from the new pipe. This is normally done with air. There are two main best practice standards that relate to the testing of sewer pipe with low-pressure air: 1.) ASTM F1417 - "Standard Test Method for Installation Acceptance of Plastic Gravity Sewer Lines Using Low-Pressure Air"; and 2.) UNI-B-6 - "Recommended Practice for Low-Pressure Air Testing of Installed Sewer Pipe."
Generally, sewer pipe plugs are placed in the section to be tested. One is equipped with an air inlet and the other has a valve to release the air. One of the plugs, or the piping that the compressor attaches to, has a gauge to record the pressure in the pipe during the test. This is done at very low pressure, around 5 psi. Never test gravity lines (or pressure lines) with high pressure air.
Pressure testing a fused PVC line prior to installation is often requested. The issue is testing to show that the fusion joints are good prior to putting the pipe into the ground. While this is a legitimate concern, in practice it is very difficult and potentially unsafe to test above ground.
The fused pipe will be hundreds of feet in length with grade changes likely. This makes filling and removing air a time consuming proposition. The pipe is unsupported allowing it to move. As pressure is added to the line after air is removed, the pipe will try to straighten itself causing movement over the ground. This movement is difficult to control and given the weight involved of a water filled line, an unsafe situation can develop. Finally, the end hardware must be secured above grade for such a test. If there is air in the line and the end hardware is not sufficiently restrained, the release of the end cap results in a fast moving projectile.
After pipe is fused above grade, it is subjected to tensile pull-in forces as well as bending. A test prior to installation therefore will miss any adverse effect of these. The only meaningful pressure test is one done after installation, on a line that is ready for acceptance into the system.