Butterfly Valves in Pump Suction 1

[JJPellin]

There is a major push to install remote isolation valves on the suction lines to many of our pumps. This is necessary for safety. In order to try and hold costs down, the piping project managers are pushing hard to get me to accept butterfly valves for the larger sizes. These valves are sometimes installed in the common suction line before it tees off to the two pumps. However, often the isolation valve is located just downstream of the existing gate valve used for pump isolation. This would place the butterfly valve within 2 or 3 pipe diameters of the suction flange. I know this is a bad idea. I know it will reduce NPSH margin, increase turbulence and encourage vibration. What I am looking for is some real world experience from other engineers that I can use as a basis to resist this effort. Has anyone had a reliable pump turn into a "bad-actor" after a butterfly valve was installed? Has anyone been forced to go back at substantial expense and replace butterfly valves in order to address chronic reliability problems? Any examples or experiences related will be greatly appreciated.

 

[ccfowler]

The placement of a butterfly valve close to a pump inlet adds a distorting influence in the flow pattern entering the pump whether or not there are NPSH or suction energy issues. These distortions of the flow pattern can induce vibration problems in the pump.

One of the most significant problems with butterfly valves is their propensity to make messy flow distributions worse. (If the flow pattern is not distorted, a fully opened butterfly valve will add relatively little distortion to the flow.) The main precaution that I would want to take is to pay close attention to nature and configuration of flow disturbing elements leading into the butterfly valve.

Although a butterfly valve is usually considered as passing full flow when it is well short of being fully opened, it will serve as a distoring element for the downstream flow pattern for any position other than fully open.

For example, if the butterfly valve must be located downstream from an elbow, I would make sure that the shaft of the butterfly valve is aligned parallel to the pipe leading into the elbow, that the butterfly is truly aligned along the pipe axis when fully open, and that the valve is located as close to the elbow as may be practical. This would minimize the influence of the butterfly valve on the flow pattern.

The reasoning for this is as follows. An elbow tends to produce opposing symmetric swirl patterns in the flow leaving the elbow. This alignment of the butterfly minimizes its potential influence when fully opened. Any other orientation of the butterfly would tend to augment and distort the turbulence in the flow downstream from the valve. Locating the butterfly valve close to the elbow maximizes the length of straight pipe after the valve to allow maximum calming of the flow pattern before the pump.