Recommended Criteria for pipe sizing

1. City water pipe sizing
   • Use “Pipe sizing for water” calculator
   • Size piping for water velocity of 3 fps < V < 7 fps (feet per second).
   • Calculate total pressure drop to verify the initial water pressure is adequate to provide minimum required pressure at the farthest  fixtures.

2. Pipe sizing for under pressure drain lines  from equipment
   • Use “Pipe sizing for water” calculator.
   • Size piping for water velocity in the range 3 fps < V < 6 fps, with minimum pipe size = equipment drain outlet size.

3. Hydronic Systems pipe sizing, closed loop chilled water, closed loop heating hot water, and open loop cooling tower
   • For systems requiring compliance with ASHRAE 90.1 (energy compliance), use the “Pipe sizing per ASHRAE 90.1 – 2019, energy Standard” calculator.
   • For ASHRAE non-compliance systems use the “Pipe sizing for water” calculator using the following criteria.
   • Add 20% to the calculated pressure drop for closed loop systems to account for aging.
   • Add 40% to the calculated pressure drop for open loop systems for aging and piping erosion.
   • For pump discharge pipe:
     • For piping smaller than 8”, use pressure drop ≤ 4 ft wc/100 ft, with water velocity V ≤ 7 fps.
     • For piping 8” and larger, use pressure drop ≤ 2 ft wc / 100 ft, with water velocity V ≤ 10 fps.
   • For pump suction pipe:
     • For boiling water use pressure drop < 1 ft wc / 100 ft.
     • For non-boiling water use water velocity < 5 fps and pressure drop < 2 ft wc / 100 ft.
   • For hydronic systems using glycol instead of water, use the pipe sizing for glycol calculator using recommended criteria for water.

4. Steam and Steam Condensate pipe sizing
   • For Steam distribution, Size steam distribution piping using “Pipe sizing for saturated steam” calculator.
   • Use the following criteria for different steam system operating pressure:
     • For saturated steam up to 15 psig pressure, use pressure drop of 1 psi / 100 ft of pipe with total pressure drop not to exceed 4 psi.
     • For saturated steam between 15 psig and 30 psig pressure, use pressure drop of 2 psi / 100 ft of pipe with total pressure drop between 5 to 10 psi.
     • For saturated steam between 30 psig and 50 psig pressure, use pressure drop up to 5 psi / 100 ft of pipe with total pressure drop between 10 to 15 psi.
     • For saturated steam between 50 psig and 100 psig pressure, use pressure drop up to 5 psi / 100 ft of pipe with total pressure drop between 15 to 25 psi.
     • For saturated steam greater than 100 psig pressure, use pressure drop up to 10 psi / 100 ft of pipe with total pressure drop between 25 to 30 psi.
     • Verify the steam velocity in piping not to exceed 8000 fpm (feet per minute).
     • Determine the pressure drop for components other than piping (i.e. control valves, strainers,…) and verify delivered steam pressure to the end user is adequate.

• Steam condensate piping,
• depending on the condensate system setup (Dry-Closed, Open (Vented)  Dry, Wet) use one of the followings as applicable.
  • Dry-Closed Condensate System;
    • This is the most common condensate return system setup. The condensate will flow in a horizontal or sloped pipe to a condensate collector tank (atmospheric or  under pressure similar to a deaerator). There will be a two-phase flow (steam / vapor & liquid condensate) in the pipe.
    • Use the “Pipe Sizing for Dry – Closed Steam condensate system” calculator.
    • Size pipe using pressure drop between 1/16 psi/100 ft to 1 psi/100 ft  of pipe. Pressure drop > 1 psi / 100 ft of pipe can result in flash steam velocity inside pipe to exceed 7000 fpm that is not recommended by ASHRAE. 
    • Use collection tank pressure for the condensate pipe pressure for calculations. For example, use zero psig pressure for the condensate pipe for an atmospheric condensate collection tank or the tank actual operating pressure (say 5 psig for a Deaerator) if under pressure. 
    • Select the design criteria (pressure drop, pipe diameter) to result a velocity not to exceed 7,000 feet per minute.
    • For more information see 2017 ASHRAE FUNDAMENTALS HANDBOOK, Chapter 22 section 3.3 Table 37 and figure 21C.
    • And as a side note, the pressure in the condensate pipe at the steam trap outlet location must be estimated and used as the back pressure for sizing and selection of the steam trap. The estimated pressure will be the pressure at the condensate collection tank plus total pressure drop through the piping (based on the selected pressure drop (psi/100 ft of pipe) and total equivalent pipe length) plus pressure drop through other components like valves, strainers, etc.
  • Wet Condensate System;
    • In this set up the condensate line is full of liquid. The pushing force to move the condensate is from the condensate head (difference in column of liquid of the inlet / outlet pipes). The condensate is collected in a vented atmospheric pressure tank.
    • Size the pipe based on the available condensate head. Use the “Pipe Sizing for Wet Steam Condensate system” calculator.
    • For more information see 2017 ASHRAE FUNDAMENTALS HANDBOOK, Chapter 22 section 3.3 Table 36 and figure 21B.
  • Open (vented) Dry Condensate System;
    • In a vented dry condensate system the pipe is vented at different locations close to where condensate enters the pipe. The flash steam is vented to the atmosphere from the pipe. The pipe is sloped down in flow direction and contains the liquid flowing at the bottom of the pipe and vapor or air above. The pipe is partially filled with liquid. The pipe diameter is calculated using Manning Equation based on the pipe slope.
    • Use the “Pipe Sizing for open (vented) dry Steam Condensate system” calculator. The condensate for this system is collected in a vented/ atmospheric tank.
    • Recommended piping slope is between 1/16 to 1/2 inches / foot of pipe.
    • For more information see 2017 ASHRAE FUNDAMENTALS HANDBOOK, Chapter 22 section 3.3 Table 35 and figure 21A.
  • Vent pipes;
    • For sizing vents for steam systems (i.e., condensate return line vent, condensate collection tank vent,…)
    • Use the “% flash steam generation from steam condensate” calculator.
    • Determine the lbs/ hr of flash steam.
    • Size vent pipe for steam velocity ≤ 3000 fpm (feet per minute)

5. Compressed Air pipe sizing
   • Use “Pipe sizing for compressed air” calculator.
   • Size piping for compressed air with total pressure drop of < 2 psi.
   • Verify:
     • Total pressure drop (P1 – P2) < 10 % of P2.
     • The demand pressure at the user is acceptable.
     • Air velocity in mains < 30 fps to avoid moisture and debris carry over.
     • Air velocities in branches < 25 fps to avoid moisture and debris carry over.

6. Natural Gas pipe sizing
   • Use “Pipe sizing for fuel gas” calculator.
   • Size piping for natural gas based on total pressure drop < 10% of the inlet pressure.
   • Verify that the demand pressure at the user is acceptable.

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