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How Does a Direct-Operated Pressure Regulator Work? (Part 2)

In Part 1 of How does a Direct-Operated Pressure Regulator Work, the purpose of pressure regulators, the history of their invention, and how their components function were discussed. Also reviewed was how pressure regulators don’t perfectly fulfill their function of maintaining a constant outlet pressure due to an attribute called droop. Part 2 concentrates on two other attributes, lock-up and boost, which also factor into a regulator’s accuracy.

 

Lock-up

If the house’s flow demand decreases to zero, the regulator needs to stop supplying gas to the house by stopping all gas flow through its orifice. As demonstrated in Part 1, the only way the regulator’s internal parts will move in any direction is if the diaphragm senses a change in outlet pressure. When flow demand decreases to zero, the outlet pressure will begin to rise since the regulator is supplying more gas than the house is using. The subsequent increase in upward force on the diaphragm from the higher outlet pressure will move the valve plug upward until it seals off flow through the orifice. The regulator adjusted its gas supply to match the change in demand. The amount of increased pressure above setpoint needed to achieve shutoff is called lock-up. Lock-up is an important regulator characteristic because it affects how well the pressure regulator fulfills its reason for existence: maintaining a constant outlet pressure.


 

Boost

The performance curves shown so far were typical for direct-operated regulators with control lines sensing a point downstream of the regulator. These performance curves show that as flow increases, the regulator fails to maintain setpoint due to droop and the performance eventually falls below an acceptable pressure. This next section focuses on internally sensing units, which are more typically used. Internally sensing direct-ops not only experience droop; they also have a counteracting force called boost that can cause the performance to exceed accuracy requirements on the high end. Boost can be used to counteract the effects of droop by extending the flowrate at which the regulator maintains accurate pressure control.

Direct-ops with internal registration sense pressure inside the valve body which is a turbulent location with high velocity. Pressure and velocity have an inverse relationship so where velocity is highest at the orifice, pressure is lowest (called the vena contracta).  Making setpoint on an internally sensing direct-op is performed exactly like an externally sensing unit: at low flow, the adjusting screw is rotated until the outlet pressure gauge reads the intended setpoint. The difference is that the regulator is not sensing the pressure at the downstream pressure gauge. Instead, it is measuring the pressure inside the body. As flow demand increases, so does velocity at the sensed point, causing an increasing discrepancy between the pressure at the regulator’s sense location (at the vena contracta) and the downstream pressure feeding the house. Consequently, the internally-sensing regulator is tricked into opening farther due to this false low pressure than if it had been sensing pressure through an external control line. This increased flow can translate into improved accuracy.


 

As demonstrated in Part 1, a regulator only moves when it detects a change in outlet pressure. Whether the direct-operated regulator senses through an external control line or through a sensing tube in the body, it is going to react identically to a given change in sensed pressure. For example, when our externally registered unit sensed an outlet pressure of 8 psig, the amount the valve plug moved away from the orifice allowed 275 scfh flow. The internally registered unit below will do the exact same thing; however, the accuracy of the pressure inside the body isn’t important. The downstream equipment is what requires accurate pressure control. An internal-sensing regulator is not controlling downstream pressure and it has no idea what the downstream pressure even is because it isn’t sensing it.

Boost is the term used to describe the elevated downstream pressure control of internally-sensing direct-operated regulators. Boost can be beneficial for extending how much of the regulator’s performance curve falls within the published accuracy. When designing a regulator, a great deal of effort is spent on placing the sensing tube at the optimal location so that the boosting effect will counteract the droop effect but not boost too much. Below is an example of an internally registered unit that boosts so much that it almost exceeds the upper accuracy bound. For ±20% accuracy, the maximum flowrate published would be 450 scfh which is much better than the 275 scfh that would be published for the same regulator with an external control line.

Boost explains why some published “capacities” are very surprising for internal sensing direct-operated regulators. From one inlet pressure to another, the turbulence in the body could create radically different amounts of boost, causing drastic changes to the flowrate at which the regulator’s performance exceeds the published accuracy limits.