What Is a Tie-Point? Interface Definition

A tie-point is a defined physical location where two separate piping systems, scopes of work, or contract packages connect. It establishes the boundary of responsibility between parties; typically between an existing plant and a new facility, between two engineering contractors, or between different construction phases of the same project. Tie-points are documented on P&IDs, piping plans, and a dedicated tie-point register to ensure that both sides of the interface match in terms of size, rating, material, orientation, and location.

Types of Tie-Points

Type	Description	Example
Brownfield tie-point	Connection to an existing, operational piping system	New process unit connected to an existing plant header
Greenfield tie-point	Connection between two new systems designed by different parties or built in different phases	Two EPC packages meeting at a battery limit
Utility tie-point	Connection to plant utility systems (steam, cooling water, air, nitrogen)	New unit receiving steam from an existing boiler
Inter-area tie-point	Connection between different plant areas or units within the same project	Pipe rack transition between Unit 100 and Unit 200
Instrument/electrical tie-point	Signal or power interface between systems	Not piping-specific, but coordinated alongside piping tie-points

Tie-Point Documentation

Document	Content
Tie-point register (list)	Master list of all tie-points: ID number, line number, size, rating, material, flange type and facing, elevation, coordinates, responsible party on each side
Tie-point drawing	Detailed sketch or isometric showing the exact connection arrangement, dimensions, and orientation
P&ID	Tie-points are shown with a flag or symbol indicating the interface boundary and reference to the connecting system
Piping isometric	Each isometric terminates at the tie-point with a reference to the connecting isometric or system
Interface agreement	Formal document between contractors or between contractor and owner defining responsibilities at each tie-point

Tie-Point Coordination Requirements

Parameter	What Must Be Agreed
Location (coordinates and elevation)	Both parties must use the same survey datum and coordinate system
Pipe size and schedule	NPS, wall thickness, and OD must match exactly at the connection point
Flange type, rating, and facing	Welding neck, slip-on, or other; ASME class; RF, RTJ, or FF; must be identical on both sides
Material and pipe class	Material grade and corrosion allowance must be compatible
Gasket and bolting	Both parties must agree on gasket type and bolt specification for the connecting flange
Orientation	Flange bolt-hole orientation (straddling the centerline or on the centerline) must be specified
Insulation and tracing	Insulation thickness and tracing continuity across the tie-point must be coordinated
Test boundary	Define which party is responsible for hydrostatic testing the tie-point joint

Pro Tip

During FEED (Front-End Engineering Design), issue a preliminary tie-point register as early as possible. Tie-point mismatches discovered during construction are among the most expensive rework items on brownfield projects. Early coordination (including a joint 3D model review) prevents misalignment in size, elevation, and material at the interface.

Tie-points are verified during pre-construction surveys and piping inspections to confirm that the physical arrangement on both sides of the interface matches the approved tie-point register before any connection work begins.

Read the full guide to piping engineering