When a flowing stream of gas is subject to a change in velocity (either speed or direction), forces arise which are the reaction to the change in momentum of the stream. This is particularly important in valve design where the position of a moving element may be affected.
The direction in which the momentum force acts is always opposite to the acceleration which is imparted to the flow stream. The magnitude of the force may be calculated by using the momentum Lohm Laws which apply to air at near room temperature.
EXAMPLE: Where a gas changes direction.
The momentum force of 0.2 lbs. in this example must be added to the force produced by static pressure on the plate (0.1 in.2 x 10 psi = 1 lb.) to give the total force on the plate.
EXAMPLE: Where a gas changes speed.
The momentum force of 0.05 lb. in this example must be subtracted from the force produced by static pressure on the plate (0.1 in.2 x [100-50] = 5 lb.) to give the total force on the piston.