A safety valve is a protective valve that prevents the medium pressure in pipelines or equipment from exceeding the specified value by discharging the medium out of the system. As an automatic valve, it is commonly used in boilers, high-pressure vessels, or pipelines to prevent explosion accidents caused by excessive pressure. Safety valves are mainly categorized into pilot-operated and direct-acting types. This article focuses on the basic operation of pilot-operated safety valves.

A pilot-operated safety valve combines a main valve with a self-operated auxiliary safety valve called a pilot valve, where the latter controls the opening and closing of the former. Pilot-operated safety valves can be classified as flow-through or non-flow-through, as well as pop-action or modulating types. For explanations of these pilot types, refer to Figure 1 and Figure 2.

Figure 1: Pop-action pilot safety valve.

Figure 2: Modulating pilot safety valve.

The pilot valve is driven by the pressure of the pressure-sensing system and uses this pressure to control the closing force of the main valve disc. Increasing system pressure raises the closing force until the pressure drives the pilot valve to open. When the pressure reaches the specified set point, the process medium is discharged through the main valve.

A pop-action pilot configuration causes the main valve disc to "pop" from the closed position to 100% open. When the system returns to normal pressure from an overpressure state, the main valve disc reseats due to medium flowing through the pilot valve to the top of the disc (upper chamber of the main valve).

As shown in Figure 3, system pressure at the main valve inlet is transmitted to the upper chamber of the main valve via interconnected pipelines through the pilot valve. This equalizes the pressure on top of the disc with the inlet pressure at the main valve seat (bottom). Since the area of the disc top is larger than that of the sealing surface, the area difference generates a net downward pressure, keeping the main valve tightly closed.

Figure 3: Main valve closed (normal position).

Figure 4 shows that as inlet pressure increases, the pilot valve piston rises, isolating the main valve inlet medium from the upper chamber. Simultaneously, the pilot valve opens an exhaust port to release medium from the upper chamber.

Figure 4: Pilot valve open (relief position).

When the pressure load above the main valve disc is removed, the upward pressure from the inlet exceeds the downward pressure above the disc, allowing the main valve disc to lift off the seat. The valve opens to relieve system pressure.

Figure 5: Main valve relieving.

When the main valve relieves and reduces inlet pressure to the pilot valve’s closing pressure, the pilot valve piston closes the exhaust port. Concurrently, the pilot valve’s inlet passage reopens, allowing medium to re-enter the upper chamber. When pressure in the upper chamber equals the inlet pressure, the downward force from the area difference between the piston surfaces closes the main valve.

The modulating pilot valve operates similarly to the pop-action type but adds the ability to maintain a proportional system pressure above the main valve disc, enabling modulation. When system pressure rises to the set pressure, the closing force is reduced by exhausting through the pilot valve, and medium is discharged through the main valve. However, the actual lift of the main valve disc depends on system overpressure rather than instantly "popping" to 100% open. This "modulating" action reduces medium loss and emissions, improving operational efficiency.

Figure 6 illustrates how system pressure at the main valve inlet is transmitted to the upper chamber via interconnected pipelines through the pilot valve, equalizing the pressure on top of the piston with the inlet pressure at the seat (bottom). Due to the larger area of the piston top compared to the seat sealing area, the area difference creates a net downward pressure, keeping the main valve closed.

Figure 6: Main valve closed (normal position).

Figure 7 shows that as inlet pressure increases, the pilot valve piston moves to isolate the main valve inlet from the upper chamber. The pilot valve opens an exhaust port, releasing medium from the upper chamber to the bottom of the regulator piston. The regulator piston has a smaller top area, which is constantly subjected to main valve inlet pressure. When pressure from the main valve’s upper chamber acts on the bottom of the regulator piston, a net upward force is generated (since the pressures are equal at this point, and the lower area is larger than the upper area). After the regulator releases some medium from the upper chamber, inlet pressure on top of the regulator piston pushes it closed. At this stage, residual medium remains in the upper chamber, with pressure controlled by the area difference in the regulator. Since pressure in the upper chamber has not dropped to atmospheric pressure, the main valve only partially opens at the set pressure. The regulator piston remains closed until inlet pressure increases further, forcing the main valve disc to lift. During this process, the regulator piston can release additional pressure from the upper chamber as needed to achieve the required lift within a 10% overpressure range.

Figure 7: Modulating position.

As inlet pressure increases further, the net upward force on the main valve increases, allowing more pressure to be relieved. The disc achieves full lift (full capacity) within 10% overpressure above the set pressure (see Figure 8).

Figure 8: Main valve fully open.

When the valve relieves and reduces inlet pressure to the pilot valve’s preset closing pressure, the pilot valve piston closes the exhaust port. Simultaneously, the pilot valve’s inlet passage reopens, allowing inlet pressure to re-enter the upper chamber. When pressure in the upper chamber equals the inlet pressure, the downward force from the piston pressure difference closes the main valve.

Advantages of Pilot-Operated Safety Valves vs. Spring-Loaded Valves. Pilot-operated safety relief valves (SRVs) offer several advantages over spring-loaded SRVs, including:

• Both the main and pilot valves of pilot-operated safety valves can achieve a seal at up to 98% of the set pressure, ensuring zero leakage under normal operating conditions even in demanding high-pressure applications.

• Compared to spring-loaded valves, pilot-operated valves provide greater seat sealing force, making them ideal for maintaining seals at high operating pressures. Operating near the maximum allowable working pressure (MAWP) helps optimize system performance.

• Full-bore pilot-operated safety valves deliver higher capacity than standard-ported valves of similar size, allowing customers to reduce costs by using smaller pipelines.

• During overpressure, modulating pilot-operated valves relieve only the required capacity (not the full rated capacity), enabling users to calculate pipeline losses based on actual system demand rather than rated flow, thereby reducing inlet pipeline pressure loss.

• On-site test ports allow operators to functionally test the pilot-operated safety valve while it remains installed and protects the system from unexpected overpressure.

• A dual-pilot option minimizes unplanned shutdowns for maintenance; a backup pilot can be used while the primary one is serviced, allowing scheduled maintenance.

• The unique design of pilot-operated safety valves connects the pilot to the main valve via interconnected pipelines and supports various accessories, including manual relief valves, filters, check valves, differential pressure switches, pilot lift testers, and remote-mounted pilots.

While pilot-operated SRVs offer advantages, they have limitations. For dirty media, filters of varying capacities and types, or specialized dirty-service accessories, can isolate critical components (regulators, main valve upper chamber, pilot exhaust/intake passages) from contaminants. However, in severely dirty environments where interconnected pipelines may clog, pilot-operated SRVs may not be optimal.

Pilot-operated SRVs require time for system pressure to propagate from the main valve inlet to the upper chamber via pipelines. During plant startups with extremely rapid pressure rises, pressure in the upper chamber may not match the inlet pressure, resulting in insufficient closing force. This can cause the main valve disc to lift and leak. Design modifications, such as adding springs in the upper chamber to increase downward pressure, can mitigate this, but spring-loaded valves may be preferable for applications with excessively fast pressure rises if such remedies are impractical or uneconomical.

Like spring-loaded valves, pilot-operated SRVs are widely used across industries, including power generation, refining/petrochemicals, chemicals, oil and gas midstream/upstream, and pulp and paper. Unique applications include high-pressure services, emission reduction in high operating pressure systems, offshore drilling and deep-well wellhead platforms, and services involving air/gas, liquids, steam, two-phase flow, or multi-condition operations.

(This article was previously published in Valve Magazine by Gardner Business Media and Control Valve Information magazine.)