Description of the basic regulator
Cracking threshold calculation
Fig. 2 Basic regulator
- Pa = Ambient pressure
- Sm = Diaphragm area
- Sc = Valve area
-Δ Pm = Inspiration suction
- Fr = Spring force -HP = High pressure The equation then becomes: (ΔPm x Sm) - (ΔPm x Sc) = Fr + (HP x Sc) - (Pa x Sc) Or: ΔPm (Sm-Sc) = Fr + (HP-Pa) Sc In fact Sc is negligible before Sm and Pa before HP. We can deduce:
Taking into account K the levers reduction ratio: (When existing)
ΔPm is the cracking threshold
Fig. 13 Cracking threshold calculation
1) The suction required to unseat the valve decreases with High Pressure (H.P). This forces the regulator to be sufficiently hard when the pressure is high so that it does not pass in constant flow when it is low.
2) The negative pressure is inversely proportional to the diaphragm area, which leads to pressure reducing valves of large dimensions.
- Practical example:
Sc = 0.04 cm²; Fr = 2 decaN; K = 40, Sm = 65 cm².
1) HP = 210 bar
ΔPm = [2 + (210 x 0,04)] / 65 x 1/40 of or ΔPm = 4 millibars (4 cm of seawater) 2) HP = 15 bar ΔPm = [2 + (15 x 0,04)] / 65 x 1/40 of or ΔPm = 1 millibar (1cm of water)
If H.P. decreases the inspiration effort decreases.
Practically these figures are affected by the shape and the nature of the material which constitutes the diaphragm as well as by the mechanism friction.
We have just studied the regulator static operation considering that the air did not circulate. In reality, as soon as a valve unseats, the air starts moving, which profoundly changes the pressures and forces that come into play. Recent studies of these phenomena have led to the most dramatic improvements of the regulators performances. Without entering into the highly complex technique of fluid mechanics, we will evoke some of the phenomena that are most involved in the operation and strongly influence the regulators morphology. Notes: In a pipe the air speed can not exceed the sound speed. Consequently, by simplifying, the maximum flow rate in a pipe of given section is equal to the product of its section by the sound speed. Example: In a pipe of 1 cm2 the flow rate can not exceed 1960 liters / minute. Pressure drops When the air flows inside a mechanism, it encounters obstacles that cause turbulence that slows down the passage of air causing "pressure drops". A pressure drop is a Pressure Difference (DDP). It is equal to the product of the dynamic resistance (R) by the flow rate (D) in cubic meters per second. DDP = R xD The greater the obstacle, the greater air velocity and the greater the losses. A simple pipe, by friction on its walls, a filter, a valve even opened, a constriction between two chambers, can cause pressure drops. In order to avoid these disadvantages, the cross-sections of the air must be increased, the surfaces inside the pipes and regulators must be polished with rounded sharp corners, the shapes studied so as to avoid turbulence. However, in a regulator, the main pressure drop is that caused by the inlet filter. This can easily be seen by trying to inhale a regulator that is not connected to a high-pressure air source. Venturi effect (See figure 14) When a gas escapes from the end of a pipe, it drags the molecules of the surrounding air by friction, causing a depression behind the pipe. This effect is all the more important as the velocity of the air at the outlet of the pipe is high.
Fig. 14 Venturi effect
In a regulator, as soon as the injector tube begins to flow, the vacuum sucks the membrane and tends to maintain the valve in continuous flow. To avoid this, one or more calibrated orifices are pierced laterally in the injector tube so as to provide the amount of air just necessary to avoid the suction under the diaphragm. These calibrated orifices are often replaced by fixed or movable deflectors which direct the air in order to stabilize the operation. Cooling When a gas extend, it cools. The temperature can thus fall well below zero degrees. Water vapor in the air can condense then freeze and cause blockage of the mechanism causing a free flow. On the "Cold page", we will find a detailed development of the influence of cold on regulator operation and possible remedies.