What Dimensions Should You Confirm for a Custom Knife Gate Valve?

Nominal pipe size alone is not enough to order a custom knife gate valve. A valve can match the stated line size and still fail to fit because its face-to-face length, bolt pattern, port opening, or actuator envelope differs from the installation. The dimensions must describe both the piping interfaces and the space the valve needs to operate and be maintained.

For a replacement, the most reliable starting point is an approved drawing of the existing valve, checked against the piping and the available space. For a new installation, the piping layout and manufacturer’s proposed drawing must be coordinated before fabrication. In either case, identify which dimensions are fixed and which can be adjusted; a custom valve cannot correct every conflict between the pipe, supports, and surrounding equipment.

Start with the piping connection, not just the valve size

Record the nominal pipe size and the actual pipe outside diameter and wall thickness where they affect the connection. Nominal size is a designation, not a complete description of the opening or the flange. Two installations identified by the same nominal size can have different flange standards, pressure designations, bolt patterns, or pipe internal diameters.

For a flanged connection, provide the applicable flange standard, pressure designation, nominal size, and facing type for each side. Do not assume that a valve described as fitting a particular pipe size will fit every flange used on that line. If the flanges are nonstandard or their identification is uncertain, send a dimensioned drawing or measurements of:

• Flange outside diameter and mating-face details.
• Bolt-hole quantity, hole diameter, and bolt-circle diameter.
• Bolt-hole orientation relative to the valve centerline.
• Inside opening and any projection or recess at the mating face.

Also identify whether the valve is to be installed between flanges with through-bolts, or connected using threaded holes or another arrangement specified by the design. These details affect bolt length, accessibility, and the manufacturer’s connection design. If each side of the line has a different interface, document both rather than treating them as interchangeable.

Flange measurements are a useful cross-check, but they do not establish the pressure rating of an unidentified flange. The connection standard, material, service conditions, and the valve’s own pressure limits need to be confirmed separately.

Measure the face-to-face length at the actual mating surfaces

Face-to-face length is the axial distance between the valve’s two mating faces. In a fixed piping run, it determines whether the valve can occupy the available opening without moving the pipework. Measure between the relevant flange faces or use a verified drawing; distinguish this dimension from the overall width of the valve body.

Where the existing valve is still installed, the visible gap may include gasket thicknesses or other components. Do not simply pass that measured gap to the manufacturer as the required valve face-to-face. Show the complete stack-up: flange face to flange face, number and type of gaskets, spacers or adapters, and the intended valve mating surfaces. The manufacturer can then establish the valve dimension on a consistent basis.

Record whether the available opening is fixed or whether piping adjustments are planned. A drawing should state the required dimension and tolerance, rather than just a rounded tape measurement. If the line cannot be opened safely for direct measurement, use existing drawings as a starting point and arrange verification during an appropriate shutdown.

Face-to-face length can be particularly sensitive in a replacement. A shorter body may leave an unaccounted gap; a longer one can prevent installation or impose stress if the flanges are forced together. Neither problem should be resolved by assuming that gasket compression will absorb a substantial mismatch.

Confirm the port opening and its alignment

The valve port is the opening through which the process medium passes. State the required port diameter or shape and compare it with the mating pipe’s internal diameter. A nominally full-bore description should be supported by actual dimensions, especially if the line contains a liner or has a nonstandard internal opening.

A smaller port can create a restriction or a ledge at the connection. A larger opening is not automatically appropriate either: it must remain compatible with the body, seat, flange opening, and pressure design. For solids-bearing or viscous service, the transition between pipe and valve deserves particular attention because an offset or abrupt step can influence material buildup and wear.

For round ports, provide the required diameter and the location of the port centerline relative to the flange bolt pattern. For rectangular or other noncircular openings, specify width, height, corner geometry where relevant, and orientation. If the pipe or chute is not centered on the valve connection, show the offset explicitly on a drawing rather than describing the opening size alone.

The internal passage may not be visible or measurable with the valve installed. When the existing valve is being replaced because of wear or blockage, a removed-component drawing or inspection can clarify whether the original port geometry is still suitable. The replacement should not automatically reproduce a worn opening.

Allow for the complete operating envelope

Overall height is not a single dimension for every knife gate valve. The required space depends on the actuator and whether the stem or gate moves beyond the body during operation. Ask for the envelope in both fully open and fully closed positions, measured from a defined reference such as the pipe centerline or bottom of the valve body.

For a handwheel-operated valve, include the handwheel diameter, the space needed to turn it, and any stem extension. For a pneumatic or hydraulic cylinder, check the cylinder’s overall length, projecting rods or covers, and space for ports and fittings. For an electric actuator, include the actuator housing, cable entry, local controls if fitted, and any manual override.

Clearance should be checked in all directions, not only vertically. A valve may clear the ceiling on paper but interfere with a nearby wall, structural member, cable tray, or adjacent valve once fittings and access are included. Note the intended mounting orientation and show nearby obstructions with distances from the pipe centerline. Do not assume that rotating an actuator or installing the valve in another orientation is acceptable without the manufacturer’s agreement.

Operating clearance is different from maintenance clearance. Packing adjustment, actuator removal, gate servicing, and fastener access may require additional space. Ask the manufacturer to identify the components that must be withdrawn and their removal direction. If overhead removal is impossible, establish whether an alternative service procedure or a different layout is needed before the order is released.

Check bolt access, supports, and installation movement

A matching bolt circle does not guarantee that the valve can be installed. The body, actuator mounting, or nearby piping can obstruct bolt heads and nuts. Provide the available clearance around the flanges, especially where bolts must be inserted from one side or where stud lengths are constrained.

For a valve installed between existing flanges, consider the space required to insert the body without damaging flange faces or gaskets. The installed gap and the space needed during assembly are not necessarily the same. If the piping cannot be spread or a neighboring component cannot be removed, discuss the installation sequence with the manufacturer before fixing the design.

Large or actuator-heavy assemblies may also require support. Give the available support locations and any restrictions on loading the pipework. The manufacturer should provide the valve assembly weight, center of gravity where needed, and approved lifting or support points. A support arrangement must not obstruct gate travel, maintenance access, or flange fasteners.

Include seat, liner, and gasket interfaces in the dimensional review

External dimensions can match while the sealing surfaces remain incompatible. Identify the mating flange facing and the proposed gasket arrangement, including any limitations imposed by the valve body or liner. A raised face, flat face, or projecting liner changes which surfaces meet and where compression is applied.

On lined or resilient-seated designs, request a section drawing showing the position of the liner and sealing surfaces at each connection. Confirm whether separate flange gaskets are required and what dimensions they must have. Do not add, omit, or substitute gaskets solely to make up a face-to-face difference; the correct arrangement depends on the valve design and the manufacturer’s instructions.

Where the process line is lined, show the liner’s internal diameter, thickness, and extent at the flange face. This helps identify interference between the valve port, pipe liner, and gasket before the parts are assembled.

Use a common drawing reference and agree on tolerances

Measurements are most useful when their reference points are unambiguous. A marked-up sketch should identify the pipe centerline, flow direction where relevant, upstream and downstream sides, flange faces, and the datum used for height and clearance dimensions. State the units throughout; if drawings from different sources use inches and millimeters, avoid mixing rounded conversions with original measurements.

Separate required dimensions from reference dimensions. For example, a fixed flange-to-flange opening may be a hard limit, while the actuator’s position may be negotiable. Conversely, a low ceiling can make the maximum operating height nonnegotiable even if the pipe spool can be modified. Marking these constraints helps the manufacturer determine where a design change is possible.

Dimensional tolerances also matter. A face-to-face value without an allowable variation leaves uncertainty about fit, particularly in a rigid existing line. Ask the manufacturer to state the manufacturing tolerances on the approval drawing and compare them with the available installation allowance. Do the same for port alignment, bolt-hole positions, and any interface that must mate with existing equipment.

Site measurements should be identified as measured, drawing-based, or estimated. If the existing flanges are not parallel, are misaligned, or appear distorted, record the condition and have the piping assessed. A custom valve should not be used as an assumed remedy for pipe misalignment; forcing the connection can place unintended loads on the assembly.

What to send with the custom valve inquiry

A useful inquiry combines dimensions with the operating conditions that determine whether those dimensions are feasible. Send the latest piping and existing-valve drawings, marked photographs where available, and a dimensioned sketch showing fixed obstructions. Identify the valve orientation, actuator type, required open and closed positions, and any limits on installation or maintenance access.

Include the medium, operating and design pressure, temperature, and the required shutoff conditions. These are not merely commercial details: they influence the body, gate, seat, and actuator design, which can in turn affect the dimensions. If differential pressure or flow direction matters to the specified duty, state the expected conditions and ask the manufacturer to confirm the permitted installation direction.

Before fabrication, review an approval drawing that shows the connection dimensions, face-to-face length, port geometry, overall operating envelope, and relevant tolerances. Compare it against both sides of the piping and the surrounding space—not only against the old valve’s nameplate. Record any agreed changes, including responsibility for adapters, gaskets, fasteners, and supports, so that the manufactured valve and the installation plan are based on the same dimensions.