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.
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.
After determining the tie-in configuration, evaluate the optimum routing of the connecting piping from a flexibility standpoint. The shortest distance between two points generally requires the least amount of piping, but this practice may be unwise from a flexibility standpoint. Added flexibility may be necessary to reduce the pipe loads on sensitive equipment such as compressors, fans or pumps. A piping jog, loop or flexible connector may offer a convenient way to reduce stress and deflection at the tie-in point or at the flanges on installed equipment.
Available software facilitates analysis of piping systems that are subject to pressure, temperature and dynamic reaction loads such as those caused by pressure relief valves. Expansion joints, hangers, anchors and guides can also be evaluated to optimize the piping support system.
For precautionary reasons, issue a line break permit to address specific details of the procedure prior to construction. The purpose of the permit is to define:
- Location of the line break.
- Applicable design documents.
- Fluid in the line.
- Safety equipment requirements.
- Hazards associated with handling the materials last in the line.
- Special line washing, purging or flushing requirements.
- Valve locking and tagging requirements.
- All personnel involved or affected by the procedure.
Installation of the connecting piping is simplified if provisions are made in the piping for isolating the tie-in point. Piping installation can then occur without disrupting the process.
When a tie-in point on a line cannot be isolated or the plant or process cannot be shut down to accommodate the line break, a procedure known as “hot tapping” is required. This is frequently used to break lines containing steam, natural gas, water or other utilities, which must flow uninterrupted on a daily basis. This procedure results in the installation of a lockable tie-in valve while the line is pressurized.
Some essential equipment for the procedure includes hot tap fitting, full open and lockable gate valve, hot tap machine (hydraulic or air-driven), cutter and pilot assembly, tapping machine housing, a power unit (hydraulic or air-driven) and hose. Basic hot tapping steps include:
- Weld the hot tap fitting onto the line.
- Install the full open gate valve on the fitting.
- Inspect and pressure test the valve and fitting.
- Install the hot tap machine on the valve.
- Bore the line.
- Retract the boring bar, then close and lock the valve.
- Depressurize the hot tap machine and remove it.
- Clean the work area.
Hot tapping is a unique, specialized procedure. Only companies experienced in this technique should be considered to perform this procedure. Often, these companies perform other tasks such as line stops, repairs, cleaning and valve replacement.
Following the hot tapping procedure, installation of connecting piping valves and fittings can proceed in much the same manner as though existing isolation provisions were already in the line. Once installation is complete, conduct a pressure and leak test of the newly installed piping with the tie-in valve remaining in the closed and locked position. With satisfactory pressure test completion, the new tie-in piping is now ready to be activated by unlocking and opening the tie-in valve.