Gate valves are used in piping systems to start and stop fluid flow. They work by lifting or lowering a round or rectangular gate into the flow path to control flow. Gate valves come in two main designs – resilient seat and metal seat. Resilient seat gate valves have a gate that presses against soft seats to achieve tight shutoff. The resilient seating surfaces can create bubble-tight isolation while being gentle on the gate. This makes them well-suited for water systems and other applications that need drip-tight shutoff.
Resilient Seat Gate Valve Construction
Resilient wedge gate valves have a gate that moves up and down in metal guides called side tracks. When open, the gate fully retracts from the flow path to provide full, unobstructed flow. The gate has seating surfaces on both sides covered with an elastomer. Common elastomers used include EPDM, NBR, and rubber. When closing the valve, the gate lowers between the resilient valve seats, creating a tight seal to stop flow. The elastic seals absorb the force of the descending gate, preventing damage.
The valve body is commonly ductile iron or cast steel. Ductile iron has good strength and corrosion resistance while being lighter weight. Other body materials like stainless steel or bronze are also used depending on the application. The stem, gate guides, and fasteners are usually stainless steel for strength and corrosion resistance.
Key Benefits of Resilient Seat Gate Valves:
- Tight shutoff – Resilient seated valves achieve bubble-tight shutoff, preventing leakage even at high pressures.
- Low operating torque – It takes less force to open/close the valve due to less friction between the gate and seats.
- Longer service life – The seats protect the gate from impacts, reducing wear and extending valve life.
- Bi-directional flow – Resilient seats allow flow in both directions making the valves suitable for reversible flow.
- Low maintenance – The seats eliminate galling and gunking common with metal-on-metal seats.
Resilient seat gate valves are available in sizes from 2″ to 48″ and pressure classes from 150 to 600 psi. They meet or exceed standards like API 609 and MSS SP-70. Typical applications include municipal water systems, irrigation, chemical processing, and power plants.
Large Size Gate Valves
For controlling very large pipes or high pressure flows, large diameter gate valves are required. These heavyweight valves come in NPS sizes ranging from 16″ to 48″. Big size gate valve weigh anywhere from 650 lbs for a 16″ class 150 valve up to 9,000 lbs for a 48″ class 600 valve. They are predominantly used in the oil & gas, chemical processing, district heating, and power industries.
The main types of large gate valves include:
- Rising stem – The stem rises up out of the valve when opening. This shows the valve position.
- Non-rising stem – The stem does not rise or lower. Valve position is not externally apparent.
- Expanding gate – The gate grows wider when closing to achieve better sealing.
- Double disc – Two parallel discs seal instead of a solid gate. Good for dirty services.
- Knife gate – Gate slides horizontally rather than rising vertically. Often used for slurry.
- Through conduit – Flow path goes straight through the gate rather than at right angles.
Large gate valves may contain special design features like body reinforcements, large stem diameters, roller guides, gear actuators, and bypasses to handle the high pressures and forces involved. Ensure valves meet applicable ASME and API standards for the service conditions.
Underground Gate Valves
Underground or buried gate valves are designed for installation beneath the earth in pipeline systems. They allow pipelines to be shut off during maintenance or emergencies without excavating. Underground gate valves come in flanged or mechanical joint ends for connecting to buried pipe sections. Their protective coatings and sealed stem designs prevent groundwater infiltration and corrosion.
There are two main types of underground gate valves:
Non-Rising Stem (NRS) – The stem and gate don’t rise above the valve, so no protective stem casing is needed. A shaft rotates the stem threads through a bushing. NRS valves are simpler but give no indication of valve position.
Rising Stem (RS) – The stem visibly rises from the valve, showing its open/closed status. A stem casing extends to ground level, protecting the exposed stem. RS valves allow position indication but the stem casing can leak over time.
Underground gate valves have flanged joint ends in sizes matching ANSI/AWWA C110/C153 pipe flanges. Typical laying lengths including the valve, joints, and operating stem are 6 to 8 feet. Bury depths range from 3 to 10 feet depending on climate freezing depth. The valve, joints, and stem casing get coated for corrosion protection against soil and water. Common coatings include epoxy, fusion-bonded epoxy, zinc, and polyethylene sleeves. A square operating nut connects to the valve stem at ground level. Underground gate valves meet standards like AWWA C500 and API 6D for reliable performance.