Piping tie-ins: the basics

Making these connections is inevitable during plant expansion. Here are a few ways to minimize costly disruption during this procedure.

By Peter Y. Burke

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Piping tie-ins are unavoidable in plants where expansion is a current or future factor. Steam, condensate, compressed air, dust collection, vacuum and process lines are just a few types of plant piping likely to be modified by tie-ins to increase capacity. Here are a few ways to minimize costly disruption of plant operation when tie-ins are inevitable.

Plan and schedule New piping design should include allowances for future expansion. This can be accommodated without major expense. Consider the following:

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  • Increase design pipe sizes to the next larger diameter if velocity limits permit.
  • Provide caps or blind flanges at the end of a pipe run where a brief tie-in outage can be tolerated.
  • Provide valves with blind flanges when and where a future tie-in outage cannot be tolerated.
  • Provide adequate access and maintenance space for future connecting piping.

Existing piping design may provide limited flexibility for future piping tie-in accommodations. However, take advantage of future scheduled outages and add piping flanges and valves where future tie-ins are inevitable. This will enable future construction to proceed without disrupting plant and process operations.


The piping and instrumentation diagram (P&ID) is a useful means to define piping tie-ins. The P&ID does not locate the tie-in physically. However, it defines the functional relationship of the tie-in within the piping system, which is the initial step in the design process. Establish a numbering system for tie-ins, i.e., “T-XXX,” which can be used as a reference on design documents until the project is completed. An indication of “new” and “existing” piping also provides clarity.

A tie-in list can also be useful for estimating the cost of construction and for scheduling work well in advance of the actual piping design activity. Often, the list will include details such as the estimated length, material designation, insulation and coating requirements and reference drawings. Some level of complexity can be assigned to the length to account for miscellaneous fittings and related variables in the cost estimate.

Finally, orthographic and isometric piping drawings should indicate the piping, fittings, valves, material, weld and testing requirements that pertain to the tie-in. Often, the tie-in references are deleted from the “as-built drawings” at the close-out phase of the project.

Design considerations

The plant and process considerations are usually obvious. Immediate or projected increases in utility or process flow rates are the most frequently cited reasons for tie-in piping. Accommodating the need for these increased demands often results in additional capital equipment.

After the process requirements are determined and the tie-in piping sized, consider isolation provisions. It is rarely feasible economically to shut down the plant or process during tie-in piping installation. Alternately, a short-term, scheduled outage may be sufficient to break the line and install a tie-in connection. After the connection is in place, the piping can be fabricated and connected “off-line” conveniently and economically.

During the interim period, a blind flange can be installed and the process resumed. However, this will require another brief outage to remove the blind flange and connect the tie-in piping. This can be avoided by installing a shutoff valve upstream of the blind flange to provide safe, long-term isolation. Subsequently, the blind flange can be removed, the connecting pipe installed, and the valve opened after the tie-in is completed.

Pay particular attention to installing a shutoff valve that is adequate for temporary “end-of-line service” by considering these factors:

  • Valve seals must withstand full rated pressure on one side while unpressurized on the other side in the closed position.
  • Valves must withstand a pressure and leak test.
  • Valves must be bidirectional so they can be installed with either end open to atmosphere.
  • Valves must be installed with a lockout device and comply with OSHA Zero Energy requirements.

The pressure drops across a tie-in valve (in the fully opened position) should not be excessive.
Provide a “drainable spool piece” downstream of the shutoff valve.

The purpose of the spool piece is two-fold. First, the drain valve provides a means to check for leakage of the shutoff valve seals. Second, the drain valve relieves pressure buildup when the blank is removed in the event the shutoff valve leaks. Provisions can then be made to deal with the faulty valve. This allows the tie-in piping to be connected safely.

A variety of valves are adaptable to end-of-line shutoff service. These include ball, gate, butterfly and plug types. Carefully compare the service conditions with the valve manufacturer’s specifications to determine suitability for both the prolonged shutoff and open positions.

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