Piping Engineering / Design Blog.



Posted by Antony Thomas at Thursday, November 17, 2011


The objective of the steam distribution system is to supply steam at the correct pressure to the point
of use. It follows; therefore, that pressure drop through the distribution system is an important feature.
One of the most important decisions in the design of a steam system is the selection of the
generating, distribution, and utilization pressures. Considering investment cost, energy efficiency, and
control stability, the pressure shall be held to the minimum values above atmospheric pressure that
are practical to accomplish the required heating task, unless detailed economic analysis indicates
advantages in higher pressure generation and distribution.
The piping system distributes the steam, returns the condensate, and removes air and noncondensable
gases. In steam heating systems, it is important that the piping system distribute steam,
not only at full design load, but also at partial loads and excess loads that can occur on system warmup.
When the system is warming up, the load on the steam mains and returns can exceed the
maximum operating load for the coldest design day, even in moderate weather. This load comes from
raising the temperature of the piping to the steam temperature and the building to the indoor design

Energy Considerations
Steam and condensate piping system have a great impact on energy usage. Proper sizing of system
components such as traps, control valves, and pipes has a tremendous effect on the efficiencies of
the system.
Condensate is a by-product of a steam system and must always be removed from the system as
soon as it accumulates, because steam moves rapidly in mains and supply piping, and if condensate
accumulates to the point where the steam can push a slug of it, serious damage can occur from the
resulting water hammer. Pipe insulation also has a tremendous effect on system energy efficiency. All
steam and condensate piping should be insulated. It may also be economically wise to save the
sensible heat of the condensate for boiler water make-up systems operational efficiency
Oversized pipe work means:
• Pipes, valves, fittings, etc. will be more expensive than necessary.
• Higher installation costs will be incurred, including support work, insulation, etc.
• For steam pipes a greater volume of condensate will be formed due to the greater heat loss.
This, in turn, means that either:
• More steam trapping is required, or wet steam is delivered to the point of use.
In a particular example:
• The cost of installing 80 mm steam pipe work was found to be 44% higher than the cost of 50
mm pipe work, which would have had adequate capacity.

The heat lost by the insulated pipe work was some 21% higher from the 80 mm pipeline than it
would have been from the 50 mm pipe work. Any non-insulated parts of the 80 mm pipe would
lose 50% more heat than the 50 mm pipe, due to the extra heat transfer surface area.
Undersized pipe work means:
• A lower pressure may only be available at the point of use. This may hinder equipment
performance due to only lower pressure steam being available.
• There is a risk of steam starvation.
• There is a greater risk of erosion, water hammer and noise due to the inherent increase in steam
The allowance for pipe fittings:
The length of travel from the boiler to the unit heater is known, but an allowance must be included
for the additional frictional resistance of the fittings. This is generally expressed in terms of
‘equivalent pipe length’. If the size of the pipe is known, the resistance of the fittings can be
calculated. As the pipe size is not yet known in this example, an addition to the equivalent length can
be used based on experience.
• If the pipe is less than 50 metres long, add an allowance for fittings of 5%.
• If the pipe is over 100 metres long and is a fairly straight run with few fittings, an allowance for
fittings of 10% would be made.
• A similar pipe length, but with more fittings, would increase the allowance towards 20%.
4.3 Selection of pipe size
There are numerous graphs, tables and slide rules available for relating steam pipe sizes to flow
rates and pressure drops.
To begin the process of determining required pipe size, it is usual to assume a velocity of flow. For
saturated steam from a boiler, 20 - 30 m/s is accepted general practice for short pipe runs. For major
lengths of distribution pipe work, pressure drop becomes the major consideration and velocities may
be slightly less. With dry steam, velocities of 40 metres/sec can be contemplated -but remember that
many steam meters suffer wear and tear under such conditions. There is also a risk of noise from
Draw a horizontal line from the saturation temperature line (Point A) on the pressure scale to the
steam mass flow rate (Point B).
• From point B, draw a vertical line to the steam velocity of 25 m/s (Point C). From point C, draw a
horizontal line across the pipe diameter scale (Point D).

The following table also summaries the recommended pipe sizes for steam at various pressure and
mass flow rate.

Piping Installation
1. All underground steam systems shall be installed a minimum of 10 feet from plastic piping and
chilled water systems. All plastic underground piping must be kept at a 10 foot distance from
steam/condensate lines.
2. Install piping free of sags or bends and with ample space between piping to permit proper
insulation applications.
3. Install steam supply piping at a minimum, uniform grade of 1/4 inch in 10 feet downward in the
direction of flow.
4. Install condensate return piping sloped downward in the direction of steam supply. Provide
condensate return pump at the building to discharge condensate back to the Campus collection
5. Install drip legs at intervals not exceeding 200 feet where pipe is pitched down in the direction
of the steam flow. Size drip legs at vertical risers full size and extend beyond the rise. Size drip
legs at other locations same diameter as the main. Provide an 18-inch drip leg for steam mains
smaller than 6 inches. In steam mains 6 inches and larger, provide drip legs sized 2 pipe sizes
smaller than the main, but not less than 4 inches.
6. Drip legs, dirt pockets, and strainer blow downs shall be equipped with gate valves to allow
removal of dirt and scale.
7. Install steam traps close to drip legs.




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