The difference between soft-seal gate valves and hard-seal gate valves

The closing member of a soft-sealed gate valve is a gate plate. The motion direction of the gate plate is perpendicular to the flow direction of the fluid. Gate valves can only be fully opened or fully closed; they cannot be used for throttling or flow regulation. The gate plate has two sealing surfaces. In commonly used wedge-type gate valves, these two sealing surfaces form a wedge shape, with the wedge angle varying depending on the valve’s specifications—typically 50°, though it may be as low as 2°52′ when the medium temperature is not high. Generally, conventional gate valves tend to suffer from leakage and are prone to rusting. In contrast, soft-sealed gate valves (also known as resilient-seat gate valves), whose gate plates are made of cast iron lined with rubber, overcome the common shortcomings of conventional gate valves, such as poor sealing performance, elastic fatigue, and susceptibility to rusting.

The soft-seal gate valve is an upgraded, next-generation version of the traditional gate valve. It features a gate disc made of elastic material, which, through its slight deformation, effectively compensates for minor misalignments and achieves excellent sealing performance. Soft-seal gate valves boast notable advantages such as light operation, reliable sealing, excellent elastic memory, and a long service life. In contrast, hard-seal gate valves have gate discs made of cast iron or stainless steel, offering low opening and closing resistance and no flow obstruction—but their sealing performance is somewhat inferior. The key difference between soft-seal and hard-seal gate valves lies in the design of the gate disc’s sealing surface.

General Requirements for Soft-Sealed Gate Valves

1. The specifications and categories of gate valves shall comply with the requirements of the pipeline design documents.

2. The model number of the gate valve shall indicate the corresponding national standard number as required. If it is an enterprise standard, the relevant specifications for the model number shall be indicated.

3. The working pressure of the gate valve shall be ≥ the working pressure of the pipeline. Without affecting the price, the valve’s maximum allowable working pressure should exceed the actual working pressure of the pipeline. Under closed conditions, either side of the gate valve should be able to withstand 1.1 times the valve’s working pressure without leakage. In the open position, the valve body should be able to withstand twice the valve’s working pressure.

The standard quality of soft-sealed gate valves

1. The valve body material shall primarily be cast iron, cast steel, stainless steel, or 316L. The grade and the actual physicochemical test data for the cast iron must be specified.

2. Valve stem material: We strive to use stainless steel valve stems (2Cr13). For large-diameter valves, the valve stems should also be stainless steel-clad.

3. The nut material shall be cast aluminum brass or cast aluminum bronze, with hardness and strength both greater than those of the valve stem.

4. The material of the valve stem bushing shall have a hardness and strength no greater than those of the valve stem, and under water-immersion conditions, it shall not cause electrochemical corrosion with either the valve stem or the valve body.

5. Material of the sealing surface

① Gate valves come in various types, with differing sealing methods and material requirements;

② For ordinary wedge-type gate valves, the material of the copper ring, its fixing method, and the grinding method should all be specified.

③ Gate valve—physical, chemical, and hygienic test data for the rubber lining material of the valve disc;

6. Valve stem packing

① Since gate valves in pipeline networks are typically not frequently opened or closed, the packing material is required to remain inactive for several years without aging and to maintain its sealing performance over the long term.

② The valve packing should also ensure the *reliability* of its sealing performance when subjected to frequent opening and closing.

③ In light of the above requirements, the valve stem packing should be designed to last a lifetime or remain unchanged for more than ten years.

④ If the packing needs to be replaced, the pneumatic valve design should incorporate measures that allow for replacement under water pressure conditions.

The operating mechanism of the soft-sealed gate valve

1. When operating a gate valve, the opening and closing direction must always be clockwise to close the valve.

2. Since pneumatic valves in pipeline networks are often manually operated, the number of opening and closing turns should not be excessive; even large-diameter valves should be within the range of 200 to 600 turns.

3. To facilitate the opening and closing operation by a single person, under pipeline working pressure conditions, the maximum operating torque should be 240 N-m.

4. The operating end of the gate valve shall feature a square tenon with standardized dimensions, and it must face upward toward the ground so that people can operate it directly from the ground level. Valves equipped with handwheels are not suitable for underground pipeline systems.

5. Indicator dial for the opening and closing degree of the gate valve

① The scale lines indicating the opening and closing degree of the gate valve shall be cast onto the gearbox cover or the outer casing of the indicator dial after the direction is reversed. All scale lines shall face downward toward the ground, and the scale lines shall be coated with fluorescent powder to ensure high visibility.

② Under well-managed conditions, the material for the indicator dial needle can be stainless steel sheet; otherwise, it should be painted steel sheet. Never use aluminum sheet to make the needle.

③ The indicator dial needle should be conspicuous and securely fixed. Once the opening and closing adjustment has been accurately set, it should be locked with rivets.

6. If a gate valve is buried at a relatively deep location and the operating mechanism and display panel are situated at a distance of ≥1.5 meters from the ground, an extended lever assembly must be provided and securely fixed, enabling operators on the ground to easily observe and operate the valve. In other words, valve opening and closing operations in the pipeline network should not involve working inside wells.

Performance Testing of Soft-Sealed Gate Valves

1. When a particular valve specification is manufactured in batches, it shall be entrusted to an authoritative institution for testing of the following performance characteristics:

① The opening and closing torque of the valve under working pressure conditions;

② The number of consecutive opening and closing cycles under working pressure that can ensure tight valve closure;

③ Measurement of the flow resistance coefficient of valves under pipeline water conveyance conditions.

2. Before leaving the factory, the valve shall undergo the following inspections:

① When the valve is in the open position, the valve body shall be subjected to an internal pressure test at twice the valve’s working pressure.

② Under the valve’s closed condition, both sides are subjected to 1.1 times the valve’s working pressure without any leakage; however, for butterfly valves with metal seals, the leakage rate shall also not exceed the relevant requirements.

Internal and external corrosion protection for soft-sealed gate valves

1. Both the interior and exterior of the valve body (including the gearbox housing) should first be blast-cleaned to remove sand and rust. Electrostatic powder coating with non-toxic epoxy resin is preferred, with a coating thickness of at least 0.3 mm. When it is difficult to apply electrostatic powder coating with non-toxic epoxy resin to extra-large valves, similar non-toxic epoxy paints can be applied by brush or spray.

2. The interior of the valve body and all parts of the valve disc must be fully corrosion-resistant—first, they should not rust when immersed in water and should avoid electrochemical corrosion between two different metals; second, their smooth surfaces should reduce hydraulic resistance.

3. The hygienic requirements for epoxy resin or paint used for corrosion protection inside the valve body shall be supported by test reports from the relevant authoritative authorities. The chemical and physical properties must also meet the applicable requirements.

Packaging and shipping

1. Lightweight sealing plates should be installed on both sides of the valve for secure closure.

2. Medium- and small-caliber valves should be bundled with straw ropes and preferably transported in containers.

3. Large-diameter valves also come with simple solid packaging made of wooden strips to prevent damage during transportation.

Troubleshooting of Soft-Sealed Gate Valves:

1. Leakage at the gate valve packing.

(1) If the packing gland is too loose, evenly tighten the nuts used to compress the packing gland.

(2) If the number of packing turns is insufficient, more packing should be added.

(3) If the packing material has become ineffective due to prolonged use or improper storage, it should be replaced with new packing. When replacing, ensure that the joints between adjacent rings are staggered by 120°.

2. Between the gate valve’s gate and the sealing surface of the valve seat

(1) If there is dirt trapped between sealing surfaces, it can be washed away and removed.

(2) If the sealing surface is damaged, it should be re-ground. If necessary, it can be re-welded and machined. The ground sealing surface must be smooth, and its roughness must not be lower than 0.4.

3. If there is leakage at the connection between the valve body and the valve bonnet due to a nut that is either not tightened sufficiently or tightened unevenly, readjust it accordingly.

(1) Any damage (such as straight grooves or groove marks) on the flange sealing surface shall be corrected.

(2) If the gasket is damaged, it should be replaced with a new one.

4. The stem of the gate valve is not operating smoothly.

(1) If the packing is compressed too tightly, you should slightly loosen the nut on the packing gland.

(2) If the packing gland is improperly positioned, causing the valve stem to become stuck, evenly tighten the nuts on the packing gland to return it to its proper position.

(3) If the threads on the valve stem and valve stem nut are damaged, they should be disassembled and repaired.