Steam Calculator: How To Calculate Steam Consumption

What is Steam Consumption in a Pipe?

Steam generated by a steam boiler is not all delivered as useful steam at the point of use. Some steam condenses as the distribution system warms and as it loses heat during operation.

The main causes of the difference between generated and useful steam are:

• Condensation caused by heat loss from pipes, valves, fittings and other exposed or poorly insulated surfaces.
• Steam leaks and failed steam traps.

When steam condenses on a cooler pipe wall, it releases its enthalpy of evaporation. The resulting condensate must be drained safely.

Reducing avoidable heat loss and leakage lowers boiler fuel, make-up water and auxiliary electricity use. The actual saving depends on boiler efficiency, condensate return, water treatment and the condition of the distribution system.

Steam-main demand can be estimated for the temporary start-up load and the continuing running load. These two conditions require separate calculations.

Steam Consumption in Steam Mains

A steam main condenses steam while the cold pipe warms and while the hot pipe loses heat to its surroundings. The warm-up load is temporary. The running load continues after the system reaches normal temperature and depends strongly on insulation and ambient conditions.

Use the warm-up condensate rate when checking initial drain and steam-trap capacity. Final trap and boiler sizing must also account for warm-up pressure, drainage-point count, safety factors, process demand and manufacturer capacity data.

What is Warm-Up Load?

After a cold shutdown, steam must heat the pipe, flanges, valves and fittings from their starting temperature towards the saturated-steam temperature at the operating pressure.

The warm-up load represents the temporary steam flow that condenses while supplying this heat. This temporary load usually exceeds the pipe’s later running heat-loss load.

The condensation rate is normally highest during warm-up. Drains, separators and steam traps must remove that condensate under the actual start-up pressure.

Follow the site’s approved start-up procedure and warm the system slowly enough to limit thermal and mechanical stress. A controlled warm-up can provide these benefits:

• Fewer leaks caused by rapid thermal expansion
• Lower maintenance demand
• Longer pipe and fitting life
• Lower risk of water hammer.

What is Process Plant Running Load?

The running load is the continuing steam demand during normal plant operation. For a steam main, it includes the steam condensed by heat loss after the pipe reaches operating temperature. It is usually lower than the warm-up load, but it is not zero and depends on pipe size, temperature, insulation and ambient conditions.

How to Warm-Up a System?

A small bypass valve installed in parallel with the main isolation valve can admit a controlled flow during warm-up. The arrangement must follow the plant’s design and operating procedure.

The permitted warm-up time and pressure determine the required bypass-valve capacity. The valve may be manual or automatic, but it must be selected and operated for the service conditions.

A full-flow main valve is often difficult to control close to its seat during warm-up. Where the approved design provides a bypass valve, use it in accordance with the start-up procedure.

Figure 1 shows a separator upstream of the main and bypass valves. Removing entrained condensate helps deliver dry steam and reduces premature wear of the valve trim.

Allowing sufficient warm-up time can provide these benefits:

• Lower thermal stress in the pipework
• Safer, more controlled operation
• Lower start-up demand on the boiler

The estimated steam flow needed to heat a pipework system depends on:

• Total mass of pipe, flanges, valves and fittings
• Specific heat capacity of the material
• Temperature rise from the initial temperature to operating temperature
• Usable enthalpy released by the condensing steam, normally the evaporation enthalpy for dry saturated steam
• Allowed warm-up time

In this idealised warm-up estimate:

• m_s: Mean steam-condensation rate, kg/h
• W: Total mass of pipe, flanges, valves and fittings, kg
• T_s: Saturated-steam temperature at the stated pressure, ^oC
• T_amb: Initial pipe or ambient temperature, °C
• C_p: Specific heat capacity of the pipe material, kJ/(kg·^oC)
• h_fg: Enthalpy of evaporation at the operating pressure, kJ/kg
• t: Warm-up time, minutes

Note: In this simple estimate, steam flow is inversely proportional to warm-up time when all other inputs stay fixed. Increasing the time from 6 to 12 minutes halves the calculated mean rate. Real start-up procedures must also control pressure, drainage and thermal expansion.

Example: calculate the mean warm-up condensation rate for a 30-minute start-up

Use these inputs:

• Carbon-steel pipe nominal size: 150 mm
• Steam pressure: approximately 15 bar(g)
• Saturated-steam temperature (T_s): approximately 201^oC
• Enthalpy of evaporation h_fg: approximately 1,933 kJ/kg
• Initial ambient temperature (T_amb): 20^oC
• Pipe length: 150 m
• Specific heat capacity C_p of steel: 0.49 kJ/kg^oC
• Warm-up time t: 30 minutes
• Total mass W: pipe plus flanges, fittings and valve
• Pipe network: 10 pairs of PN40 flanged joints and one isolation valve

Table 1: Typical masses of Schedule 40 steel pipe, flange pairs and flanged isolation valves.

Use Table 1 to calculate W from each steam-main item:

• 150 mm Schedule 40 steel pipe: 28.2 kg/m
• 150 mm PN40 flange pair: 28 kg per pair
• 150 mm PN40 isolation valve: 88 kg each

The total metal mass is 4,598 kg. Substitution in the warm-up equation gives a mean condensation rate of about 422 kg/h for the stated 30-minute period: