Sunday, October 6, 2013

PressFit Piping Systems Installation and Advantages

The Pressfit piping system is an innovative, rigid, self-restrained mechanical joining method for schedule 5 or lighter weight lightweight stainless steel and carbon steel pipe. This proprietary mechanical pipe joint is designed for use in small-bore piping systems, NPS ¹⁄₂ (DN15) to NPS 2 (DN50). The Pressfit system may be applied to any service that is compatible with the piping materials, the gasket material, and the temperature range of the system, unless prohibited by the manufacturer’s instructions.

Typical applications would include building-services piping, potable water, fire protection, heating and cooling, industrial processes, process cooling and heating systems, plant utilities, and vacuum systems.

Joint Concept

Pressfit joint
Fig. 1: Pressfit joint
The Pressfit joining system concept is illustrated in Fig. 1 The left side of figure shows the pipe fully inserted into the Pressfit fitting in the ‘‘unpressed’’ condition. The right side of figure shows a cross-sectional view of the Pressfit joint in the ‘‘pressed’’ condition. Note that the pressing operation indents the Pressfit fitting and pipe, thus providing the mechanical restraint required to resist pressure and external loads that try to separate the pipe. The O-ring seal has also been compressed to provide the pressure-boundary seal of the joint. Additionally, the final pressed shape of the Pressfit joint provides resistance to torsional movement.

Industry Specification, Codes and Product Testing

Pressfit fittings and pipe meet the requirements of the following specifications, codes, and standards:

● Pressfit carbon steel products meet the requirements of ASTM A53 Grade A and A135 Grade A. Pressfit stainless steel products meet the requirements of ASTM A312 Grade 316/316L and ASTM A269 Grade 304/304L.

● Pressfit products meet the requirements for use in piping systems designed to comply with ASME B31.1, B31.3 and B31.9 piping codes. Pressfit products are qualified for use in these systems by the following paragraphs:
● Codes and standards that have approved or listed Pressfit products are:
  • Underwriters’ Laboratories—UL Listed 
  • Underwriters’ Laboratories Canada—ULC Listed
  • Factory Mutual—FM Approval
  • Southern Building Code Congress International, Public Safety Testing, Evaluation
  • Service Inc.—SBCCI, PST, and ESI Report No. 9535
● International Conference of Building Official and Uniform Mechanical Code — UMC, ICBO-ES Report No. 5079

● Building Officials and Code Administrators—BOCA Evaluation Services Inc. Listed Report No. 93–3 Cat. 22 and Cat. 15

● National Fire Protection Association—NFPA 13

● Underwriters’ Laboratories—ANSI/NSF-61 listed for stainless steel potable water service

System Pressure and Temperature Rating

The Pressfit pipe joining system, when installed in accordance with the manufacturer’s instructions, is rated as follows:

● Pressfit joints are rated for 300 psi (2065 kPa) when used in general service or process systems.

● Pressfit joints are rated for 175 psi (1200 kPa) for all fire protection services.
Table A: Thermal service conditions
● The maximum and minimum continuous service temperatures for Pressfit joints are defined by the selection of the O-ring seal which is compatible with the system fluid. Thermal service conditions are shown in Table A. A comparison of the maximum allowable design pressure of the Pressfit joint to an ASME Class 150 joint over the temperature range from ambient to the maximum continuous service temperature of the Pressfit joint is shown in Figs. 2, 3 and 4.

Fig. 2: 316/316L Stainless Steel PressFit
Stainless Steel VIC-Press 304
Fig. 3: Stainless Steel VIC-Press 304
Fig. 4: Carbon Steel PressFit

Joint Installation

Pressfit pipe fittings are designed to be installed on square cut, plain-end pipe. No special pipe-end preparations are needed. Pressfit joints are made using generally accepted pipe fitting techniques with the addition of the following requirements: 

● Each pipe end must be marked by measuring back from the end to establish an insertion or witness mark. This mark should be highly visible and extend for at least 180 of the pipe circumference. The insertion depth should be measured and marked as shown in Table B.
Pressfit insertion mark depth
Table B: Pressfit insertion mark depth
● The marked pipe end should be fully inserted into the Pressfit fitting completely to the pipe stop. The insertion or witness mark should be adjacent to the end of the Pressfit fitting. The Pressfit pipe fitting should  be squared to the pipe and pressed onto the pipe using the proper pressing jaw and Pressfit tool.

System Installation

As with all piping systems, a Pressfit system must be properly installed to provide the system performance envisioned by the piping designer. At minimum, the following installation requirements should be considered:

● System Support: Like all other piping systems, pipe joined with Pressfit joints requires support to carry the weight of the piping system, system fluid, and other system equipment. As in other methods of joining pipes, the support or hanging method must be adequate to eliminate undue stresses on joints, piping, and other system components. The suggested maximum span between supports for Pressfit piping systems is shown in Table C.
Table C: Suggested maximum span between supports
● Thermal Expansion and Contraction: As with all rigid piping systems, piping installed utilizing Pressfit joints must be reviewed by the piping designer to assure proper allowances are incorporated into the piping system design to eliminate undue stresses from thermal expansion or contraction. The use of flexible mechanical coupling-type expansion joints is highly recommended for this service. If installation of flexible mechanical joints is not possible or desired, the designer is encouraged to use single-leg (Z-shaped) or dual-leg (U-shaped) expansion compensation loops as shown in Figs. 5 and 6.
Fig.5: Z-shaped expansion compensator
Fig. 6: U-shaped expansion compensator pipe with fittings
For calculated piping movement, (delta)L, the minimum expansion compensate leg length L may be determined by using Figs. A9.7 and A9.8.
Fig. 7: Z-shaped expansion compensator Graph
Fig. 8: Z-shaped expansion compensator Graph
As a result of thermal expansion and contraction of pipe, Pressfit joints may be subjected to torsional or rotational movement. Rotational angles must be limited to a maximum of 5 .

Advantages of Pressfit

The Pressfit pipe joint was conceived to provide a fast, clean, and cool method of installing lightweight carbon and stainless steel piping systems. Advantages provided by using Pressfit are listed as follows: 

● The Pressfit piping system, with its lower weight, lack of required pipe-end preparation, along with ease and speed of pressing joints, will provide a lower final cost installation to the contractor and owner than the same size carbon or stainless steel system installed by threading, flanging, or welding.

● Due to the design of the Pressfit fitting, piping designers can take advantage of the full-rated pressure capability of the Pressfit fitting across the allowed temperature range of the selected O-ring material. Pressure derating with an increase in metal temperature is not a factor in Pressfit systems as compared to a flanged system. Refer to Figs. 2, 3, and 4 for comparison.

● Pipe used in piping systems utilizing Pressfit joints has thinner nominal wall thickness than Schedule 40 pipe used in most applications where Pressfit should be considered. This difference results in significant increases of flow area and less pressure drop in Pressfit piping systems, compared to systems designed utilizing Schedule 40 pipe. A tabulation of these factors is shown in Tables D and E.
Friction Loss
Table D: Friction Loss
Flow Area
Table E: Flow Area
● When considering carbon steel Pressfit and Schedule 40 piping from an internal corrosion perspective, the Pressfit system provides adequate performance when used in closed-loop service where water treatment is maintained or introduction of oxygen into the system is limited to periodic testing or system makeup. In Table A9.6 and Fig. A9.9, the corrosion resistance ratio (CRR) of Schedule 5 and Schedule 40 carbon-steel Pressfit pipe are compared. The corrosion resistance ratio (CRR) is a method, established by Underwriters’ Laboratories in 1970, by which to compare the effective wall thicknesses for various pipes. The effective wall thickness is the minimum thickness remaining at any point within a system which has exposure to both internal and external corrosion.

For Schedule 5 pipe, the effective wall thickness is the minimum allowed by the applicable ASTM standard and for threaded Schedule 40, it is the minimum remaining thickness under the first exposed thread. Threaded Schedule 40 is used as the baseline and has a CRR of 1. Piping with a CRR greater than 1 will have an effective wall thickness greater than threaded Schedule 40. As can be seen in the table above, Schedule 5 Pressfit pipe has an effective wall thickness greater than threaded Schedule 40 in sizes up through NPS 1¹⁄₂ (DN 40). This is normally adequate to assure long system life.

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