Bypass on large gate valves: why it is needed and how to specify it
Large gate valves often carry a small bypass line with its own valve. It protects the main valve and the pipeline and lets the valve open without excessive effort.
TAZ Engineering Team · · 5 min
Why a bypass
When a large closed gate valve holds a high differential pressure, the force pressing the wedge onto the seat is proportional to the seat area and the pressure difference, and it becomes very large. Opening is hard and unseating wears the seats. Opening the bypass first equalises pressure on both sides so the main valve opens easily.
Steam warm-up and line filling
On steam lines the bypass admits steam slowly to warm the downstream section, giving condensate time to drain and reducing water hammer risk. On water and heating mains it allows slow filling and pressurisation.
Construction
A bypass consists of connections on the body on each side of the wedge and a small globe or gate valve between them. Connection locations and sizes for steel valves are described, for example, in MSS SP-45. A bypass can also be built in the pipework around the valve.
When to order one
Consider a bypass for DN 300 and above with high differential pressure, on steam lines and on large manually operated valves. State the need, the bypass DN and the bypass valve type in the RFQ; TAZ agrees the arrangement in the technical offer.
Checklist
- main valve DN
- maximum differential pressure
- medium
- warm-up need
- manual or actuated
- bypass DN
- bypass valve type