Valves & Vents: Microhydro Relief

Placing a valve between the intake’s pipe and the main penstock can be quite handy when you need to shut down a hydro system for maintenance or freezing weather. Any type of valve, other than a plastic slide-gate, will work. I usually place a union close-coupled just below the gate valve, so I can clear pebbles or other debris that might accumulate in the cavity at the bottom of the valve and prevent the valve from closing completely.

Sometimes, instead of adding a valve, the connector between the penstock and the intake pipe, such as a rubber compression union, can be removed to keep water out of a penstock. This is not only a hassle and slower than a valve but also problematic if the pipeline has to be shut down in a hurry. Some systems may use a rubber hose to connect the two pipes. In this case, it is important to make sure that the hose is suction-rated and has wire coil molded into the rubber to prevent collapse. A screw-apart union or a compression splice can also work to keep water out of the penstock.

At the turbine, consider using a gate valve for the main shutoff—especially for high-head systems. Compared to a ball valve, they operate slower, reducing the risk of water hammer—an effect caused by stopping a flow of water too quickly. There’s a lot of kinetic energy in water flowing down a pipe, and gradually slowing the flow to a stop will avoid the high pressures that can break or weaken a pipe and its joints. A ball or vane valve can be a better choice on lower-head applications because they create less turbulence at their typically higher-flow rates. Regardless of your system type, this rule is paramount: Be careful to shut the main valve at the turbine slowly to avoid water hammer.

Venting also should be included in any design, especially in a system that has an upper shutoff valve, because there is a chance that the intake screen could become blocked enough to collapse the penstock. The down-pipe movement of water can create tremendous suction in the pipe if its flow is stopped from above. At 200 feet of head, a pipe has greater risk of collapsing from suction than breaking from water hammer when a turbine shutoff valve is suddenly closed.

The diagram above shows an automatic air-vacuum-operated release valve. If a vacuum occurs, the valve lets the penstock drain without damage. Penstocks 4 inches and smaller should use a 1/2-inch or larger valve. Larger pipe should have a valve sized no less than one-sixteenth the diameter of the penstock.

Another method for suction relief uses a standpipe, which must be placed below the valve to the penstock and extend above the maximum level of the water source at the diversion. This keeps the penstock open to the atmosphere above the water level. Be sure to place the standpipe far enough down the penstock so that the system will have enough suction to draw through the intake instead of sucking air down the standpipe.

A standpipe can also help purge from the penstock any air bubbles mixed in with the water that result from turbulence in the intake area. Spring-loaded and manual vents are also available. Avoid the floating ball type because they tend to open when they shouldn’t, sucking air at higher-flow rates or in the event of partially obstructed intake filters. Air trapped at any high points in the pipeline can slow or stop water, decreasing or even stopping turbine output. At low heads (32 feet or less), this is not usually a problem, but air in the penstock can also lead to premature bearing failure in the turbine, or even damage the runner, when an on–off repeated pulse set up by water–airspace repetitions acts like little hammer blows hitting the runner. The higher the head and the bigger the airspace, the greater the blow to the turbine.

Another air-removal method is to use permanently installed manual valves to bleed the air from the high spots when the penstock is full but not flowing. Some hydro system owners will simply thread a screw into the penstock at the high points, and occasionally back the screw out to bleed any accumulated air.

The bottom of the system should always include a drain valve for draining all piping or bypassing the hydro plant. In cold climates, the unburied end of the penstock, valves, and hydro manifold are susceptible to freeze damage if the flow is stopped, so it is critical to keep the drains or bypass open if freezing weather is imminent. This applies to all nozzles on the hydro—not just the ones that are currently “on.” Re-jet the nozzles if necessary to  be able to open them all. If you can’t, it is better to shut the whole thing down and drain everything. Not paying attention to this can cost you several hundred dollars in freeze damage.