Both back pressure regulators and proportional relief valves are pressure control devices capable of opening to relieve system overpressure and automatically closing upon pressure recovery. However, while the back pressure regulator precisely maintains upstream pressure through a pressure regulation system, the proportional relief valve achieves approximate pressure stabilization via dynamic proportional adjustment. These devices differ in their primary functions, working principles, and installation locations, etc.

Back Pressure Regulator

 

A back pressure regulator is a pressure control device that restricts and precisely regulates upstream pressure to maintain it at a set value. Its primary function is to dynamically release excess upstream pressure into the downstream piping, ensuring the upstream pressure remains stable within the preset range. The device continuously monitors upstream fluid pressure and automatically opens to relieve excess pressure when it exceeds the set value. Upon detecting pressure returning to the designated range, the device closes to maintain the required process pressure for upstream equipment and instruments.

Figure 1 illustrates a typical application of the FITOK back pressure regulator in a system. The FITOK pressure-reducing regulator is usually installed upstream in the line, while the back pressure regulator is installed downstream to ensure the process operates under constant pressure.

Figure 1: Schematic Diagram of FITOK Back Pressure Regulator Application

• Working Principle

The pressure regulation system of a back pressure regulator consists of three core components: the loading mechanism, sensing mechanism, and control mechanism, which work together to achieve precise pressure control.

1.Structure and Function

Loading Mechanism: Applies an adjustable loading force (Fs) to the valve body via a range spring from the top.

Sensing Mechanism: Monitors inlet pressure (Fi), outlet pressure (Fo), loading force, and similar parameters in real time.

Control Mechanism: Adjusts the fluid path by sealing or separating the lift poppet and seat assembly.

2.Operational Cycle

The back pressure regulator (such as the FITOK BDGC series) controls pressure by maintaining dynamic force equilibrium at the sensing mechanism, where the poppet spring force (Fp), inlet pressure (Fi), outlet pressure (Fo), and loading force (Fs) are balanced. Figure 2 compares the open and closed positions of this type of back pressure regulator.

Opening: During overpressure conditions (Fs ＜ Fp + Fi + Fo), when the upstream pressure rises above the set pressure of the back pressure regulator, the fluid pushes the sensing mechanism upward, separating the lift poppet from the seat, and excess inlet pressure is discharged through the outlet.

Closing: During underpressure conditions (Fs > Fp + Fi + Fo), when the upstream pressure is below the set pressure, the fluid cannot push the sensing mechanism upward, and the lift poppet remains sealed against the seat, maintaining the upstream piping pressure.

Figure 2 BDGC series Back Pressure Regulator Working Principle

Proportional Relief Valve

 

A proportional relief valve opens proportionally as the upstream pressure exceeds the cracking pressure and gradually closes as the upstream pressure decreases. Its primary function is to release piping pressure by diverting fluid flow, avoiding overpressure caused by pressure spikes and ensuring the system operates within an acceptable pressure range.

Unlike a safety valve, a proportional relief valve begins to open when the system reaches the set pressure and increases its opening proportionally with further pressure rise to approximately maintain stable upstream pressure. In contrast, a safety valve opens substantially as inlet pressure reaches the set threshold and fully as pressure rises, rapidly relieving pressure to protect personnel, property, and the environment. Figure 3 shows the FITOK proportional relief valves RM and RV series.

Fig. 3: FITOK Proportional Relief Valve

• Working Principle

The proportional relief valve opens and closes automatically based on the pressure difference between the inlet pressure (Fi), the system back pressure(Fb) and the loading force (Fs), Figure 4 illustrates the working principle of the FITOK RM series proportional relief valves. FITOK RV and RM series utilize balanced stem design to eliminate the effect of system back pressure, whereas the RL series requires pre-set pressure shall be compensated by subtracting 80% of system back pressure.

Valve Closed: Inlet pressure < set pressure.

Initial Relief: Inlet pressure = set pressure; the fluid overcomes the loading force to open the valve and begins releasing fluid.

Proportional Opening: Inlet pressure > set pressure; As the inlet pressure increases, the valve opens proportionally.

Proportional Reset: Inlet pressure > set pressure; As the inlet pressure returns to normal, the valve resets proportionally.

Valve Resealing: Inlet pressure ≤ resealing pressure; the valve closes. The resealing pressure is always lower than the set pressure.

The set pressure is pre-adjusted via a spring and refers to the inlet pressure at which the valve opens and fluid begins to flow. After initial relief, the set pressure may fluctuate within a certain range. For details, refer to the Proportional Relief Valve catalog.

Figure 4: RM Series Proportional Relief Valve Working Principle

Comparison of Back Pressure Regulator, Proportional Relief Valve, and Safety Valve