PG Gland Connection Types: NPT, Socket Weld, Flanged, and Welded Options

PG Gland Connection Types and Installation

The PG compression sealing gland is the pressure boundary between the process environment and the atmosphere. It must provide a reliable, leak-free seal around the probe tube under the full range of operating pressures and temperatures while still allowing the tube to be adjusted or retracted when needed. A significant part of gland performance depends on the connection type — how the gland body interfaces with the isolation valve or nozzle below it.

This guide covers the four primary connection types, sealant selection, body material options, special gland configurations, and the torque specifications required for proper installation.

NPT (National Pipe Thread)

NPT is the most common connection type for PG glands. The gland body has a male NPT thread on its process-side outlet that threads directly into a female NPT coupling on the isolation valve or nozzle.

Characteristics

• Tapered thread creates both a mechanical connection and a pressure seal
• Available in sizes from 1/4" NPT (PG2) through 2" NPT (PG9)
• Thread sealant (PTFE tape or anaerobic pipe sealant) is applied to the male threads before assembly
• Can be assembled and disassembled with standard wrenches
• No special tooling or certification required for installation

Advantages

• Fastest and simplest installation method
• Lowest cost
• Easy field replacement — the gland can be unthreaded and replaced without cutting or welding
• Widely available fittings and adapters

Limitations

• Not recommended for services subject to severe vibration or thermal cycling, which can cause thread loosening
• Pressure rating is limited by the NPT thread engagement — typically adequate up to ANSI Class 300 but consult pressure-temperature tables for specific sizes
• Repeated assembly/disassembly can damage threads, particularly in softer alloys like Monel

Socket Weld

A socket weld connection permanently joins the PG gland body to the mating valve or fitting through a fillet weld. The gland outlet is inserted into a socket weld bore, and a circumferential weld is applied around the joint.

Characteristics

• Provides a permanent, high-integrity connection
• Rated for higher pressures than NPT (suitable for Class 600 and above)
• Eliminates the risk of thread loosening under vibration or cycling
• Requires a certified welder and post-weld inspection per the facility's welding procedure specification (WPS)

Advantages

• Superior pressure integrity — no thread path for leakage
• Excellent vibration and fatigue resistance
• Smooth bore transition with no thread crevices that could trap contaminants

Limitations

• The gland cannot be removed without cutting the weld — maintenance requires either re-welding or replacing the entire connection assembly
• Higher installation cost due to welding labor and inspection requirements
• Field replacement is more complex and time-consuming

When to Specify

Socket weld connections are appropriate for high-pressure applications (above Class 300), services with significant vibration (compressor stations, reciprocating equipment), and installations where long-term reliability without maintenance access is acceptable.

Threaded Flange

A threaded flange connection uses a standard ASME B16.5 flange with an NPT-threaded bore. The PG gland's NPT outlet threads into the flange, and the flange is then bolted to a mating flange on the valve or nozzle.

Characteristics

• Combines the convenience of NPT threading with the robustness of a bolted flange joint
• The gland can be unthreaded from the flange for maintenance without disturbing the bolted joint
• Flange ratings per ASME B16.5: Class 150 through Class 2500
• Gasket between flange faces provides additional sealing integrity

Advantages

• Bolted connection provides high clamping force and excellent leak resistance
• Easy maintenance — the flange can be unbolted to remove the entire gland-valve stack as a unit
• Compliant with facility piping specifications that mandate flanged connections

Limitations

• Higher cost than NPT alone due to additional flange, gasket, and bolting hardware
• Greater physical size and weight
• Requires proper bolt torquing sequence to ensure even gasket compression

Welded Flange

A welded flange provides the highest connection integrity. The PG gland body is socket-welded or butt-welded to a weld neck or socket weld flange, creating a permanent metal-to-metal joint. The flange is then bolted to the mating process flange.

Characteristics

• Permanent welded joint between gland and flange — no threads at the gland-to-flange interface
• Bolted flange-to-flange connection at the process interface allows disassembly for maintenance
• Highest pressure and temperature ratings available
• Requires welding and inspection during fabrication (typically done in shop, not field)

Advantages

• Maximum pressure integrity — the weakest link is the bolted flange joint, which is well-characterized and easily inspected
• Suitable for the most severe services: high pressure, high temperature, sour gas, cyclic loading
• The assembly (gland + welded flange) can be unbolted from the process for maintenance while keeping the welded joint intact

Limitations

• Highest cost of all connection types
• The gland-to-flange weld is permanent — if the gland body is damaged, the entire assembly must be replaced
• Longer lead time due to shop fabrication and inspection requirements

Sealant Selection

The sealant material packed inside the PG gland bore is the component that actually creates the pressure seal against the probe tube. Selecting the correct sealant is critical for both seal performance and service life.

Selection Guidelines:

• For the vast majority of applications below 500 deg F with no unusual chemical attack, Teflon is the default choice. It provides reliable sealing, low friction for probe adjustment, and long service life.
• When the process contains aggressive solvents (ketones, esters, chlorinated hydrocarbons) that may degrade PTFE, specify PEEK.
• For any service above 500 deg F, Grafoil is the only option. Note that Grafoil has higher friction than Teflon, so greater force is required to adjust probe position, and retraction under pressure requires more operator effort.

PG Body Materials

The gland body must be compatible with the process environment and the mating connection materials. Available body materials include:

• 303 SST — Free-machining stainless steel, suitable for non-corrosive services and shop prototypes
• 316 SST — Standard for most process applications, good general corrosion resistance
• 316L SST — Low carbon for welded assemblies, minimizes carbide precipitation in heat-affected zones
• 316 NACE — Meets NACE MR0175/ISO 15156 hardness and chemistry requirements for sour gas service
• Monel 400 — Specified for hydrofluoric acid, seawater, and brackish water services
• Hastelloy C22 — Broad-spectrum resistance for mixed acid and oxidizing chloride environments
• Hastelloy C276 — Premier choice for the most aggressive chemical environments

The gland body material should match or exceed the corrosion resistance of the probe tube and isolation valve to prevent galvanic corrosion at dissimilar metal interfaces.

Special Gland Options

Beyond standard configurations, several special options address specific application requirements:

• Counterbore / Bonda Relief — A machined relief at the bore entrance that reduces stress concentration on the sealant and improves seal life under high-pressure cycling
• Integral O-Ring — An elastomeric O-ring groove machined into the gland bore provides a secondary seal behind the primary compression sealant, adding redundancy for critical services
• Dual Sealant — Two different sealant materials stacked in the bore (for example, Teflon on the process side and Grafoil on the atmospheric side) to handle temperature gradients where the process-side temperature exceeds the primary sealant rating near the pipe but drops below it at the atmospheric end

Torque Specifications

Proper gland nut torque is essential for achieving a reliable seal without over-compressing the sealant (which increases friction and accelerates wear) or under-compressing it (which causes leakage). The following table provides recommended torque values for Teflon sealant assemblies:

Notes on torque:

• PEEK sealant requires approximately 10-15% higher torque than Teflon due to its greater stiffness
• Grafoil requires approximately 20-25% higher torque than Teflon and should be tightened in incremental steps (one-quarter turn at a time) to allow the graphite to flow evenly
• Always use a calibrated torque wrench — hand-tight or "mechanic's feel" is insufficient for pressure-rated sealing glands
• Re-torque after the first thermal cycle to compensate for sealant relaxation

For a broader understanding of how the PG gland fits within the complete assembly component stack and how different assembly types affect gland selection, see our companion guides. For additional technical detail on packing gland design, see our Conax PG packing gland overview.