What is a Companion Flange

A companion flange is a pipe fitting used to create a bolted flanged connection between a pipe and another flanged component, such as a valve, pump, vessel, meter or section of pipework. It provides a matching connection face so that two components can be drawn together with bolts while a gasket or other sealing arrangement forms a leak-resistant joint between them.

The term is commonly used for a flange intended to mate with another flange already provided on equipment or pipework. Its dimensions and pressure capability must be compatible with the opposing flange and with the operating conditions of the system. Matching the nominal pipe size alone is not sufficient because flange standards define details such as outside diameter, bolt-hole arrangement, number of bolts and facing configuration.

Unlike a permanently welded pipe joint, a flanged connection can normally be dismantled by removing the bolts. This makes companion flanges particularly useful around equipment that may need to be isolated, removed or replaced during the life of a pipeline.

The Flange Creates a Bolted Load Path Around the Joint

A companion flange does not normally seal because its metal face is simply pressed against another metal face. In many common piping arrangements, sealing is provided by a gasket compressed between the mating flange faces.

When the flange bolts are tightened, tensile force develops in the bolts and creates compressive loading across the joint. The flange distributes this load around the circumference so that the gasket can maintain contact with the sealing surfaces.

A simplified joint consists of:

1. the companion flange connected to the pipe;
2. the mating flange on the valve, pump or other component;
3. a gasket positioned between the appropriate flange faces;
4. bolts or studs passing through the aligned bolt holes;
5. nuts used to develop the required clamping force.

All of these elements interact. A correctly specified flange can still leak if the gasket is unsuitable, the faces are damaged or the bolts do not provide sufficiently uniform compression.

Excessive tightening is not a reliable solution to leakage either. Too much bolt load can damage a gasket, overstress fasteners or distort components. Assembly requirements therefore depend on flange design, gasket type, bolt material and the applicable specification.

Alignment before tightening is particularly important. Bolts should not be treated as tools for forcing substantially misaligned pipework into position. Doing so can introduce loads into the flange, connected pipe and equipment nozzles before the system is even pressurised.

This is especially relevant at pumps and other machinery. Pipework that has to be forcibly pulled into alignment with an equipment flange can transfer unwanted forces to the equipment connection.

Compatibility Requires More Than the Same Pipe Diameter

Two flanges can both be described as suitable for the same nominal pipe size and still be incompatible. The complete flange specification determines whether they can form the intended joint.

The most obvious requirement is the bolt pattern. Bolt holes on the two mating flanges must align correctly, with the appropriate number, diameter and pitch circle arrangement.

The facing arrangement must also be compatible with the intended gasket. Different flange systems can use flat faces, raised faces or other specialised sealing geometries.

Key compatibility points include:

Standards are especially important because several flange systems are used internationally. A nominal size expressed in one system does not prove dimensional interchangeability with a flange manufactured to another standard.

Pressure rating also requires more care than selecting a flange from a single pressure number. Allowable pressure can depend on material and temperature as well as flange designation. The connected equipment, gasket and fasteners must also be suitable for the service.

The lowest-rated relevant component can limit the joint. Installing a higher-rated companion flange does not increase the allowable operating pressure of a valve or other component with a lower rating.

Bore compatibility should also be considered where flow characteristics matter. An abrupt internal reduction or misalignment can create additional resistance and local turbulence even when the bolt holes align correctly.

Companion Flanges Can Connect to Pipework in Different Ways

The mating face defines how the flange joins the opposing component, but the opposite side of the companion flange has to connect to the pipe. That connection varies with pipe material and piping system.

In metallic industrial pipework, the flange may form part of a welded or threaded arrangement depending on the design. In other piping systems, a flanged adaptor or purpose-designed fitting may provide the transition between the pipe and a standard flange connection.

This distinction matters because the flange joint and pipe joint perform separate tasks. The bolts and gasket seal the interface between the two flange faces, while the pipe-side connection must independently resist the forces and pressures acting between the flange and pipe.

The method selected needs to account for:

• pipe material;
• outside diameter and wall dimensions;
• operating pressure and temperature;
• axial forces within the pipeline;
• corrosion environment;
• installation and dismantling requirements;
• compatibility with the mating equipment.

Flanged connections are commonly found at valves because the valve may eventually require removal. Pumps are another typical application because maintenance or replacement may require the suction or discharge pipework to be disconnected.

Meters, strainers and other inline equipment can use similar arrangements. In these cases, the practical advantage is not that the flange makes the equipment itself easier to repair, but that it provides a defined dismantlable interface between the equipment and fixed pipework.

A companion flange may also be used where a pipe terminates at an existing flanged connection. The flange effectively converts the pipe end into the geometry needed to mate with that component.

Gasket Compression and Bolt Loading Control the Seal

A flange joint can appear mechanically secure while still being unable to maintain a reliable seal. The condition of the gasket contact area and the distribution of bolt load are central to joint performance.

Before assembly, the flange faces should be checked for contamination and damage that could interfere with sealing. Old gasket material remaining on a face can prevent a replacement gasket from seating uniformly.

The gasket must also correspond to the flange arrangement and service. Its dimensions need to place the sealing material in the intended contact area without obstructing the bore or extending incorrectly into regions not designed for gasket compression.

During assembly, bolts are generally tightened in a controlled sequence rather than taking each bolt immediately to its final load in circumferential order. Progressive tightening helps distribute compression more evenly around the gasket.

Several conditions can produce leakage even when all bolts are present:

• damaged or contaminated flange faces;
• incorrect gasket dimensions or material;
• uneven bolt loading;
• flange misalignment;
• unsuitable fasteners;
• pipe loads acting on the joint;
• corrosion of sealing or fastening surfaces.

Joint movement can also arise from temperature changes. Pipework expands and contracts as temperature varies, potentially transmitting forces and moments into flange connections if the wider system does not accommodate that movement appropriately.

Vibration is relevant around pumps and other rotating equipment. The flange itself is not a vibration isolator. Where movement or vibration control is required, it has to be addressed by the design of the complete piping system rather than expected from the companion flange.

Flanged Connections Affect Maintenance and Pipeline Layout

The ability to dismantle a companion flange is useful only if the surrounding installation provides enough space to separate the components. Removing the bolts does not necessarily allow a valve or pump to be lifted out if rigid pipework prevents the flanges from moving apart.

Pipeline layout therefore has to consider how equipment will actually be removed.

Isolation is equally important. A flange cannot safely be opened simply because the connected equipment has stopped operating. The relevant pipe section must be placed in an appropriate safe condition for the system involved, including dealing with pressure and retained fluid.

For buried installations, access creates an additional design constraint. A flanged valve may be located in a chamber so that bolts, fasteners and the equipment itself remain accessible. Directly burying a connection that is expected to be dismantled later can remove much of the maintenance advantage of using a flange.

External conditions can affect fasteners and flange surfaces even when the internal seal remains sound. Moisture, aggressive ground conditions or corrosive atmospheres may deteriorate exposed metallic components, so material selection and protective measures need to reflect the installation environment.

Evidence around a flange joint can also help distinguish different failure mechanisms. Leakage concentrated at the gasket interface suggests a different problem from leakage at the pipe-side connection. Corroded fasteners indicate a different concern from a flange that has been mechanically pulled out of alignment.

When equipment is removed, the exposed flange faces provide an opportunity to inspect sealing surfaces, bolt condition and alignment before reassembly. A new gasket does not correct damaged faces or pipework that places the connection under significant mechanical strain, so those conditions need to be addressed before the joint is reassembled.