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Steam Main - Heat Loss - Start up and Running Condensate Rate - Help

The Steam Main Start-Up and Running Condensing Rate page provides a simple calculation system for determining the condensing rates and heat losses for steam pipes.

To use the Steam Main Start-Up and Running Condensing Rate page do the following:

Note: If you change your mind about anything such as the inputs, the format of the results or the units, simply make your changes and click Calculate again.

Please refer to the links below for further information on:
Scope
Steam Pressure
Start Up
Minimum Number of Traps Required
Condensate Capacity per Trap
Pipe Location
Nominal Pipe Size
Cp for Carbon Steel
Start Up Valve Capacity
Insulation Factors and Type
Running Condensing Rate
Running Heat Losses
Equivalent Pipe Length
Glossary



Scope

Steam distribution pipes (commonly called steam mains) and branch lines are designed to transport steam from a pressure source to the point of use. In doing so they use a little energy in (a) heating up the pipe from ambient temperature when first energised and (b) continuing to lose heat from radiation when up to working temperature.

This utility calculates the steam flowrate required to:

It also returns figures in energy terms as well as mass terms.

The utility also recommends the correct number of trapping points for the considered length of pipe, plus the size of control valve required to warm up the pipe in a controlled manner over the specified time period.

Bare pipes and insulated pipes are considered, and three different thicknesses of insulation can be compared alongside the different effects of air movement associated with pipes to be found indoors, sheltered outdoors and exposed outdoors.

Steam Pressure

Start Up

The utility calculates the maximum rate at which steam will be condensed during start-up when the pipe is at ambient temperature. It is important that this condensing rate is known because, when it is greater than the running load, it is used to size the steam traps serving the line.

During the start-up sequence, drain valves should be opened to allow the condensate to discharge to atmosphere. It is important, when the steam main is approaching normal running temperatures, to shut the drain valves to allow the steam traps to operate correctly. This procedure can be performed manually or automatically by the use of drain valves, installed parallel to the steam trap and discharging via an open-ended pipe below the valve.

Procedure for warming through steam pipework from cold. The following is as recommended by the National Industrial Fuel Efficiency Service (NIFES) of the UK in their 'Boiler Operators' Handbook'.

  1. Check that all system isolating valves, including those to branch lines. are SHUT.
  2. Check that all system drain valves are OPEN. At least one drain valve or steam trap should be fitted to each section of pipe between isolating valves.
  3. Crack OPEN the system main isolating valve at the source of live steam.
  4. SLOWLY warm through the section of pipework up to the next isolating valve to purge the pipe of air and condensate. Check that manual drains are passing condensate.
  5. Continue the warming through process until the pipe section being purged is at the working temperature and pressure.
  6. SHUT the drains in that section, and check that steam traps are operating correctly by passing condensate.
  7. SLOWLY OPEN the main isolating valve at the source of live steam to its full extent.
  8. Crack OPEN the next isolating valve and warm through the system pipework in a similar manner as from point (b) above.
  9. Continue to progressively warm through the system pipework until it has been completely purged of air and condensate and is all at working pressure and temperature.

Warning: Waterhammer

  1. The entry of steam into a pipe in which there is an accumulation of water, may suddenly condense and form a vacuum, setting the water into violent motion. The momentum of this water may be sufficient to cause extensive mechanical damage to pipework and fittings.
  2. Waterhammer may be produced in a steam pipe in which water has been allowed to collect (such as a 'dead end') by any slight variation in conditions.
  3. The sudden opening of drains may be enough to produce violent motion of the water in a pipe under steam; this is a frequent cause of waterhammer.

ALWAYS:

  1. Open a steam valve very slowly and in small increments
  2. If there is an indication of waterhammer in the surrounding pipes, SHUT the steam isolating valve at once and ensure that the pipes are being properly drained before re-opening the valve. Drain lines should be allowed to fall directly to a point lower than the steam pipe being drained. If the drain pipe rises above the steam pipe a backpressure will be formed that reduces the drainage efficiency, and steps should be taken to bypass/modify the rising line to allow condensate to gravitate to an open end below.
  3. Ensure the correct operation of steam traps in drain lines or, if necessary, keep drains open sufficiently to prevent the accumulation of water.
  4. If it is necessary to open drains on a pipe system suspected of containing water, open drains GRADUALLY.

Minimum Number of Traps Required

The utility calculates the recommended number of traps to be used on an uninterrupted length of horizontal steam main subjected to the recommended degree of fall. This is based on a maximum trapping frequency once every 30 m and a minimum of one every 50 m.

Example:
A pipe up to 50 m long will return 1 trap.
A pipe 51 m - 89 m will return 2 traps
A pipe 90 m - 119 m will return 3 traps
A pipe 120 m - 149 m will return 4 traps
A pipe 150 m - 179 m will return 5 traps
etc.

(Note: at least one steam trap should be used to drain any section of pipe between two isolation valves, even if the length between these valves is less than 30 metres).

The utility only considers horizontal pipe with no rising sections. If a rising section exists, a steam trap should be placed at the bottom of the riser to stop water building up at this point, irrespective of the pipe length.

Condensate Capacity per Trap

Trap capacity can be determined from the maximum steam load divided by the recommended number of traps. The maximum steam load can occur at start-up or during normal running, depending on the start-up time and whether the pipe is insulated. The user should check both start-up and running loads, and apply the larger of the two figures to the calculation.

If drain valves are installed in parallel with the steam traps and discharging down to grade there is no need to apply a safety factor to size the steam trap. If drain valves are NOT installed OR, the pipe is NOT insulated, multiply the trap capacity by a factor of 2.

Each trap is sized on the working differential pressure. Note that if traps are discharging to atmosphere, the trap differential pressure will be the working pressure of the main. If the traps are discharging into a condensate return line, the condensate may be subject to a backpressure, and this must be subtracted from the steam main pressure to give the differential pressure across the trap.

For Spirax Sarco traps, there is no need to apply any other safety factor. At start-up, the pressure in the steam main will be considerably less than the working pressure, and it is often thought that a safety factor should be applied because of this. It should be remembered that Spirax Sarco steam trap capacity charts show condensate rates at or near steam temperatures. At start-up, condensate temperatures will be close to ambient, and trap capacities will typically increase by a factor of 4 due to the fact that:-

  1. The trap orifice is not restricted by flash steam, as is the case at higher temperatures
  2. The density of cold condensate is typically 5% greater than condensate near to saturation temperature

Warning: Please note that many trap manufacturers quote their trap capacities in terms of cold water. Such traps will have much lower capacities when operating at steam temperatures, and it is therefore necessary to add additional safety factors. It is recommended that trap capacities at near-to-steam temperatures are checked with the trap manufacturer before making any commitment to buy or install their equipment.

Example 1:

Safety factor not required
Working pressure: 10 bar g
Start-up load: 870 kg/h
Running load: 82 kg/h
Number of traps: 6
Capacity per trap: 870/6 = 145 kg/h
Size each trap on DP of 10 bar g and 145 kg/h
Traps selected are 6 no. " TD42L

Example 2:

Safety factor of 2 is required, as no drain valves are fitted
Working pressure: 10 bar g
Backpressure: 4 bar g
Differential pressure: 6 bar g
Start-up load: 870 kg/h
Running load: 82 kg/h
Number of traps: 6
Capacity per trap: 870/6 = 145 kg/h x 2 = 290 kg/h
Size each trap on DP of 6 bar g and 290 kg/h
Traps selected are 6 no. " TD42H

Example 3:

Safety factor of 2 is required, as pipe is not insulated
Working pressure: 10 bar g
Backpressure: 4 bar g
Differential pressure: 6 bar g
Start-up load: 1026 kg/h
Running load: 570 kg/h
Number of traps: 6
Capacity per trap: 1026/6 = 171 kg/h x 2 = 342 kg/h
Size each trap on DP of 6 bar g and 342 kg/h
Traps selected are 6 no. " TD42H

Pipe Location

Three separate pipe locations are considered: situated indoors, situated outdoors but in a sheltered location; and situated outdoors in an exposed position. Pipes will lose more heat when exposed to air movement due to convection (although when lagged, the effect is reduced). Pipes located indoors are assumed not to be subject to air movement, whilst pipes outdoors in sheltered positions are assumed to be subject to air velocities up to 4 m/s, and pipes outdoors in exposed positions are assumed to be subject to air velocities over 4 m/s.

Nominal Pipe Size

This is a reference diameter given to pipes to make selection easier; either in imperial or metric terms. Pipes can be chosen from the available list, which shows the most common standards. The pipe weights will vary between standards and this is considered in calculating the warm-up times, but has no effect on the running losses.

Cp for Carbon Steel

The specific heat for carbon steel is taken as 0.49 kJ/kgK irrespective of the pipe standard.

Start Up Valve Capacity

The utility calculates the maximum rate at which steam will be condensed during start-up when the pipe is at ambient temperature. The condensing rate will depend upon the time required for the line to be brought up to its working pressure. For instance, a pipeline warmed up in 15 minutes will condense steam twice as fast than a pipe warmed up in 30 minutes. The running load is totally independent of the start-up time and considers the radiation losses from the pipeline when it is up to working temperature.

It is not the intention of Spirax Sarco to recommend correct start-up times, as these will vary relative to many factors, and is usually best known to the plant operators. The important thing is that, whatever start-up time is chosen, enough steam traps of the correct capacity are installed to cope with the maximum condensing rate. However, the utility has a minimum start-up time of 10 minutes, for safety reasons.

The main object of the utility is to determine the number and size of steam traps required to handle the start-up rate. However, it also provides a minimum valve capacity index (Kv or Cv) to match the required start-up time. This valve capacity will normally be much lower than the capacity of a line-sized isolating valve, and can therefore be interpreted in two ways.

Diagram of Start-up valve location:

Start-up valve location

Insulation Factors and Type

The utility calculates heat losses from bare pipes and pipes lagged with 50mm, 75mm, and 100mm thicknesses of insulation, each of which will give different results. To account for these differences, the calculator considers the heat losses from the bare pipe and multiplies these by a factor depending upon the insulation thickness. The insulation factors relate to preformed sections made out of calcium silicate.

Running Condensing Rate

The running condensing rate is calculated using the heat losses from the pipe. These are given to allow the operator to compare the start-up load to the running load. In many cases, the start-up load will be higher than the running load, but with long start-up times and poor insulation, it is possible that running loads can be higher than start-up loads.

Running Heat Losses

The running heat losses (kW) are calculated from the running load by taking the condensing rate and dividing this by the latent heat of the steam at its working pressure. Heat losses are included because it allows a direct comparison with the expected heat losses from other fluids flowing in similar pipelines.

Equivalent Pipe Length

Ideally, the numbers of pipe flanges and valves should be considered, as these will increase the total weight of the pipe, affecting the start-up load, whilst the extra radiation surface will affect the running load.

Should this information be available, the extra weight should be compared to the weight of the pipe, and added as a percentage to the length of pipe, and entered as an equivalent pipe length.

Should this information not be available, it is suggested that an equivalent pipe length is entered, based on the following proposition:

Equivalent pipe length:
Flanged pipe: add 20% on to the pipe length
Screwed pipe: add 10% on to the pipe length

Glossary

Please refer to the Glossary for an explanation of the terms used on the Steam Mains Start Up and Running Condensing Rate pages.