Five common steam line sizing errors
When operating a boiler for process or building heat, engineers deal with a dynamic system. Phase changes, mass and energy balances, mixed-phase flows, non-Newtonian compressible gases, changes in pressure and velocity, and production-based or seasonal load swings are characteristics of this complex system.
Across the breadth of steam systems field engineers encounter, most steam systems are far from perfect. Some problems result from inadequate design and some developed in the course of expansions or modifications. Many of the problems involve incorrect steam and condensate line pipe sizing (see “Pipe friction data”).
The effects of these errors range from noise and erosion to equipment damage, erratic process control, safety hazards, and higher fuel costs. Understanding the causes and effects of these errors can go a long way to minimizing damage and improving the reliability of the system.
Sizing errors and effects
Incorrectly sized distribution piping
In the past many process plants have used higher steam velocities, approximately 200 ft/sec, reasoning that increased pipe noise is not a problem within the process plant.
Noise is not the sole issue. Only where appreciable superheat (dry gas) is employed should velocities exceed 120 ft/sec.
Undersized distribution piping introduces problems such as:
Higher pressure drop
Insufficient steam flow to point of use
High risk of water hammer, noise, and erosion problems.
Oversized distribution piping introduces problems such as:
Unnecessarily expensive large-diameter piping, support pipe, fittings, valves, and installation labor
High heat losses from excessive condensate formation due to the greater surface area of the steam pipe
Poor steam quality caused by the formation of additional condensate.
Problems can occur when it is decided that boiler pressure can be changed without consideration of the existing pipe size. The most cost-effective method is to run the boiler at its design pressure and reduce the pressure at the point of usage.
Assume it is planned to set a 3450 lb/hr (100 hp) rated boiler at the 50-psig requirements of the process systems. The resulting steam would occupy a volume of 6.7 ft3/lb. For acceptable velocities this pressure setting would require a 4-inch steam line (Sch. 40). However, if the same boiler is set at 200 psig, steam tables show the volume reduces to 2.14 ft3/lb, requiring only a 2-inch steam pipe.
Look at the economic impact. A 200-foot, 4-inch, Sch. 40 main steam line would cost three times more than a 200-foot, 2-inch main line.
Undersized piping downstream of pressure reducing valves
Assume the boiler is set at 200 psig, 2-inch steam lines are used, and a 2-inch pressure-reducing valve (PRV) is specified to provide the 50-psig steam required by the process. However, the same 2-inch piping for the PRV inlet is used for the outlet. This is the third common problem encountered.
Velocity through the pressure reduction station must be considered when sizing fittings and components. Designers sizing these lines often fail to properly consider the significant increase in specific volume of steam with reduced pressure. If the designer had checked the specific volume at the two pressures, he would have seen that the steam at reduced pressure occupies more volume, and the outlet piping has to be a minimum of 4-inch to maintain reasonable velocity.
This error would result in much higher steam velocities downstream of the PRV and a high level of noise. The high velocity could cause premature erosion of the outlet piping and downstream components.
Undersized condensate piping downstream of traps
Steam performs its work (gives up its latent heat) and condenses back to water. This condensate is usually discharged from the steam lines at system operating pressure by a properly sized and selected steam trap.
Pipe sizing for condensate recovery takes on a new dimension: two-phase flow brought about by the larger effective volume and reduced pressure of the condensate system. When hot condensate under pressure is released to a lower pressure, its temperature must drop very quickly to the boiling point for the lower pressure. This sensible heat released from the liquid condensate causes some re-evaporation into flash steam, and the two phases are present in the condensate system.
If the condensate pipes are undersized by neglecting the presence of flash steam, the result is increased back pressure and velocity, this time with a liquid phase that could lead to water hammer. Severe water hammer can cause damage to steam system pipes, equipment, and personnel. Increased back-pressure can create problems with process equipment.
Improperly sized pumped condensate return lines
Condensate is an energy resource that should be returned to the boiler feed water system. Pumped condensate return lines in a properly engineered system only carry water. Flow rates of no more than 6 to 8 ft/sec should be used for proper pipe sizing.
Check for sizing errors
Make use of several basic principles and methods to assess a steam system for potential problems.
First, keep in mind which part of the steam system is being assessed:
Generation and distribution (gas phase)
Pressurized condensate return (mixed phases)
Pumped condensate recovery (liquid phase)
Since steam is a compressible gas; pressure and volume are inversely proportional. As steam pressure increases, the volume of space it occupies (specific volume) decreases. Steam tables and pressure/specific volume graphs (Fig. 1) are useful in quantifying the relationship between steam pressure and volume to size piping correctly.
Piping should be sized with consideration for fluid velocity and pressure drop. For velocity, the relationships among the saturated steam velocity, specific volume, and pipe diameter is:
V = (2.4QVs)/A
V = Velocity, ft/min
Q = Steam flow, lb/hr
Vs = Specific volume at flow pressure, ft3/lb
A = Internal cross-sectional pipe area, inch2
To size by pressure drop, make use of a variation of the D’Arcy Equation that calculates pressure loss due to friction for straight pipe of constant diameter for fluids of reasonably constant density.
dP = (pfLv 2 )/(144D2g)
dP = Differential pressure, psi
p = Density of fluid, lb/ft3
f = Friction factor (dimensionless, see table previous page)
L = Length of pipe, ft
v = Flow velocity, ft/sec
D = Internal pipe diameter, ft
g = Gravitational constant (32.2 ft/sec2)
Since the values for ‘f’ are complex, they are generally obtained from tables.
Constant velocity after PRVs
Downstream piping cross-sectional area must be larger by the same ratio as the change in volume. Consult steam tables for the specific volume of saturated steam for the two pressures, then calculate the ratio of the downstream pressure to the upstream pressure (always greater than 1).
Use the velocity equation to solve for cross sectional area of the high-pressure side for the design velocity and flow rate. Then multiply the cross sectional area by the ratio to figure what the cross sectional area of the low-pressure side should be. Use pipe size tables to find a pipe diameter for the pipe schedule being used.
Piping downstream of traps
Taking flash steam into consideration, condensate piping has to be sized based on two-phase flow (the presence of steam and condensate). A portion of the pipe will be occupied by flash steam and the balance with condensate.
Line velocity of no more than 50 to 66 ft/sec is recommended for condensate return lines and vent pipes. Sizing programs and nomographs are available that combine sizing calculations for the size of a flash vessel, condensate line, and flash steam vent line in one chart (Fig. 2). This can help to correctly size all two-phase flow components.
Condensate return lines
Condensate line sizing can be checked with the aid of readily available charts or programs correlating flow rate, flow velocity, pressure drop, and pipe diameter. Apply friction loss per length of pipe run, and friction equivalents (in straight pipe length) of the various tees, elbows, and other fittings in the line to these results.
Check the capacity of installed condensate pumps in relation to return line flow rates. Electric pumps are usually sized with a pumping capability of 2
Beyond pipe sizing
Correct steam pipe sizing is critical not only for energy efficiency, but also for safety. Provide good quality steam at the required demand and pressure of the user. By using the proper tools, a steam system can be sized to optimum conditions, with minimum heat loss and maintenance attention.
These sizing considerations underscore the importance of auditing a steam system periodically, especially whenever process equipment is altered, or boiler ratings or operating conditions are changed. This inexpensive process and the resulting optimization measures can preserve plant efficiency, maintainability, and safety.
Questions about steam line sizing should be directed to 800-833-3246. For more information on allowing for flash steam in piping refer to Design of Fluid Systems-Hook-Ups, Spirax Sarco, Inc., 12th edition. Article edited by Joseph L. Foszcz, Senior Editor, 630-288-8776, firstname.lastname@example.org .
Pipe friction data
Pipe size , inch 1 1/ 1 2 2 4 5 6 8-10 12-16 18-24 Friction factor , f 0.027 0.025 0.023 0.022 0.021 0.019 0.018 0.017 0.016 0.015 0.014 0.013 0.012
5 common pipe sizing problems:
Incorrectly sized distribution piping, from not optimizing steam velocity.
Oversized distribution piping, from altered boiler operating conditions
Undersized piping downstream of pressure reducing valves, from failing to consider changes in steam velocity and specific volume.
Undersized condensate piping downstream of traps, ignoring the presence of two-phase flow.
Improperly sized condensate return lines, from failure to differentiate between pressurized and pumped condensate.