Four Core Reasons Why Gate & Globe Valves Must Be Equipped with a Back Seat

1. Isolate Medium Pressure to Protect Packing and Extend Service Life

Gate and globe valves adopt **rising stem design**. When fully open, the conical/spherical surface on the stem shoulder seals tightly against the back seat in the bonnet, forming an independent metal seal that blocks the path from the valve body to the packing gland.

- Without a back seat, full line pressure acts continuously on the packing during operation, causing hardening, damage and leakage under high-temperature steam or corrosive media.

- With a back seat, the packing is barely exposed to pressure in the fully open position, greatly slowing wear and aging, reducing external leakage and frequent adjustment.

2. Allow Online Packing Replacement Under Pressure Without Shutdown

In chemical plants, power stations and high-pressure pipelines, shutdown is costly and risky. The back seat enables safe maintenance:

- Fully open the valve to engage the back seat and isolate the medium below the packing chamber.

- Loosen the packing gland, remove old packing and install new packing while the line remains pressurized.

- Avoid production losses and safety hazards from releasing flammable, explosive or toxic media.

3. Provide Dual Sealing to Meet Safety and Environmental Standards

- Packing serves as the primary dynamic seal; the back seat acts as a secondary static seal in the fully open position, providing redundant protection.

- For steam, gas and toxic chemicals, strict external leakage control is mandatory. The back seat prevents ejection even if packing fails.

- GB/T 13927 and ASME B16.34 require mandatory back seat pressure testing; valves without back seating are considered non-compliant.

4. Match Structural Characteristics (Not Required for Ball/Butterfly Valves)

Gate and globe valves have **axially rising stems** that can form a sealing surface at the top position.

Ball and butterfly valves use **rotary stems** without axial lift, so a back seat cannot be implemented and is not required by standards.

- Globe valve: Low pressure on packing when closed, but high pressure when fully open, requiring back seat pressure relief.

- Gate valve: Full line pressure acts on packing in the open position, making continuous leakage highly likely without a back seat.

Reminder

The valve **must be fully open** to engage the back seat. At partial opening, the back seat is ineffective and packing bears full pressure, leading to rapid damage.