Hydraulic Analysis Limited undertake many hydraulic studies of borehole pumping systems every year for different water authorities and consultancies. Borehole risers are often allowed to drain down to the level of the local water table when the pumps are not operating due to the elevation difference between the water table and ground level. The pumping main is normally prevented from draining back into the well through the installation of a check valve at the top of the wellhead. As a result of this, the start-up of the pumps (which are normally submersible units installed at the bottom of the wells) can be problematical as large volumes of air need to be vented out of the riser. The air is normally expelled from the riser via an air valve at the top of the wellhead and the selection of the most appropriate air valve must form a critical part of the surge analysis.
If the air valve outlet orifice is too large then excessively high surge pressures and pipe loads can be generated when all of the air has been expelled from the wellhead and the air valve closes. This also risks damage to the air valve mechanism and Hydraulic Analysis Limited are frequently called in to investigate the cause of damage to wellhead air valves due to an inadequate surge analysis being undertaken at the detailed design stage of the project. This can often be due to the installation of ‘anti-slam air valves’ which can exasperate any problems.
If the outlet orifice of the air valve is too small then the surge pressures will be low but there is a risk of building up a high air back pressure in the well during venting with the consequent danger of forcing a pressurised air pocket into the pumping main through the check valve. This air pocket can them become trapped in the pipeline with a consequent reduction in achievable flowrates and / or severe pressure fluctuations as the trapped air repeatedly expands and compresses.
In addition to the above problems on pump start-up, a power failure at the pumps during normal operation risks prolonged periods of sub-atmospheric pressures in the pumping main at the wellheads which can result in large volumes of air being drawn into the pipeline through the wellhead. These adverse conditions can be mitigated by Hydraulic Analysis Limited through modification of the wellhead air valve connecting pipework and / or the installation of a surge vessel at the wellhead.