Referenced Bypass Pressure Regulator

[wilddog27]

I've pulled apart a simple automotive fuel bypass pressure regulator that has a reference pressure(vacuum connection). It's similar to the image linked below. This regulator works in a 1:1 ratio, where every 1 psi increase in reference pressure causes the fluid pressure to increase by 1 psi. How does the regulator do this 1:1 ratio? If 1 psi is added to the reference pressure how does the ball seat move up (in the case of the linked image) enough to provide a 1 psi increase? Is there a linear relation between the ball diameter and spring force+reference pressure force?

 

[gerhardl]

You have to look at the total forces closing and opening the membrane. The membrane has (seen from your sketch) two (almost?) identical pressurized areas. Lets call them A1 (top)and A2 (underside). Your reference pressure we can call R and ingoing pressure P1. Spring pressure is S,

At startup, pressure opening the valve is: Pressure down (P1xA1) less pressure up((RxA2) + S). As long as the pressure up is larger than the pressure down the valve will remain closed, any overpressure on top will open the valve.

If you now increase (or for that case decreases) the R, the resulting opening pressure down must increase (decrease)in the same proportion if area A1=A2.

Theoretically there will be a pressure differenc between ingoing pressure (P1) and outgoing pressure (P2) from the fuel chamber in the dynamic situation (to drive the fuel through). This will not influence the way of looking at the membrane balance.

The valve in itself will act as an opening/closing valve, not as a balancing throtteling valve in my opinion. This would have requiered a needle (regulating) type seat and a more damped operation.

 

[LittleInch]

I'll go with gerhardl on this one. what you seem to be saying is that you think there is some sort of control action / feedback loop going on here and unless there is other pipeowork outside of this device, I can't see how varying the pressure in port 7 will do anything other than vary the pressure at which the regualtor opens or closes.

Or may be that's your point at which the explaination above makes sense, i.e. the spring force is the base force or maximum pressure that valve opens and then any variation in the vacuum applied to port 7 has a direct bearing on the set pressure in what is actually a variable pressure relief valve and not really a regulator. On the basis that the fuel supply is limited in flow, then if too much flows out of the valve, the fuel pressure goes down and valve closes until the pressure builds up again. For a small system this will result in a steady pressure and because the area on both sides of the diaphram are the same then you have your 1:1 ratio. If the flow rate was very low you might hear this valve "chatter" as the valve opens and close rapidly, but becasue the space betweent he ball and it's seat is quite small, this in reality is seen to control the flow and pressure. Because the valve is not really controlling pressure, only altering the set pressure at which point the valve starts to "weep", if you had a lot more flow capacity available than the valve could handle, the fuel pressure would go up, but this seems to have been sized correctly.

To work correctly the back pressure from the fuel return line also needs to be very low compared to the fuel presusre coming in or this would also act to open the valve.

Hope that makes senses - it took me a few goes to get my head around it but in fact it is very simple.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way