Stop Check Valves
In many applications the use of a combination of a stop valve and a check valve is used.
This results in two pressure containing parts, two potential leak paths and the added issues of space and especially weight.
Based on globe valves, stop-check valves have been developed in the past. By mounting a loose disk on the spindle, the disk can function as a check valve. The main disadvantage is that these valves in most cases have not been designed as stop check valves. A second disadvantage is the fact that the pressure drop over this type of valves is high, due to the change in flow direction.
If we build the start stop combination, based on two different pressure-containing bodies, the unit would be identical to this drawing. There are two pressure seal bonnets and two heavy forged valve bodies, welded together or welded in different sections of the line. In total we have to deal with three or four welds, although AVS can carry out the welds between the check valve and the gate valve in the factory.
Other disadvantages are the space requirements, the additional weight, the extra insulation needed and also the additional cost factor.
Furthermore, the maintenance and inspection costs for these combinations, have to be based on two valves.