Reservoir drainage valve actuator pressure requirements

[bellmouth]

I am selecting a butterfly valve to be installed on a submerged pipe intake at the bottom of a reservoir, and need to determine the upstream and downstream pressures of the valve so the actuator torque can be determined. The valve is a shutoff valve for the pipe, that when opened allows water to flow to a pump station. The valve is a 48" diameter valve, and the desired water velocity through the pipe when the valve is opened is 15 feet per second. The valve is located at 100 feet below the reservoir surface. If there is a pump at the downstream end of the pipe, do I factor that atmospheric pressure is just on the upstream side of the valve, and include that as part of the upstream pressure? Or assume air pressure on both sides of the valve and neglect it in the calculation, and just use the reservoir height for the upstream pressure?

 

[roydm]

I would design for the worst possible case with a vacuum on the pump suction (130ft) however a more significant number is the Breakout Torque requirement which is effected by the seal and bearing material.
Are you sure it's a butterfly and not an eccentric disk? the torque requirements for those is significantly less.
Your valve/actuator supplier should also check the calculation.
Whatever you do make sure you don't undersize it, allways better to have an oversize actuator rather than undersize.
Roy

 

[gerhardl]

I go with roydem, in my opinion he has very precisely pinpointed your problem.

My advice is to select a double eccentric butterfly valve (= valve with double eccentric disc), to keep the breakout torque at a minimum over longest possible lifetime before seal change.

Double eccentric expected lifetime for sealing, depending on make and construction up to 25 years and more. Centric valve construction in comparison: considerably less. Torque comparison between the valves if sealings hardens, worst case: centric impossible to move, double eccentric operational but not sealing properly.

With a double eccentric valve, to a suitable pressure class, European PN6 or PN10 (bar), or US 150lbs, the normal torque for the valve for water will be sufficient. Producer to advice, but valve and torque must qualify to below characteristics.

Check that the valve and sealing construction is suitable for one sided vacuum down to a necessary degree (coarse vacuum). Check that spindle, bearing and total construction is suitable for water speed larger than your expected max at full one sided pressure according to class.

For a 'free' intake valve you will probably use a submerged oil hydraulic actuator (???). It will then not cost much extra to have a safety margin of say 50% to 100% force.

If the valve is mounted in connection with an intake dam: alternative then is submerged gear on the valve, extended stem and dry mounted electric actuator with comparable safety margin. (Multiturn actuator with reduction gear to keep size down)

For dam wall mounting penstock type intake constructions (four sides thightening) is an alternative.