Steam distribution header (steam manifold)

01

Size the header, take steam from the top and drain the bottom

Spirax Sarco's boiler house tutorial says the header diameter should be calculated for a maximum steam velocity of 15 m/s at full load, because low velocity helps entrained moisture fall out. Off-takes should always be from the top of the header, so that condensate falls to the bottom and only dry steam is exported, and condensate should be removed as soon as it forms; for this, "a mechanical trap, for instance a float trap, is the best choice". If the header is the first trapping point after the boilers, the trap may be drained to the blowdown vessel rather than the feedtank, because the condensate can contain carryover. [1]

For pipework beyond the header, Spirax Sarco's pipe sizing module states that "as a general rule, a velocity of 25 to 40 m/s is used when saturated steam is the medium", treats 40 m/s as a practical limit, notes that "in longer supply lines, it is often necessary to restrict velocities to 15 m/s to avoid high pressure drops", and works an example in which 5,000 kg/h of dry saturated steam at 7 bar g needs a 130 mm bore at 25 m/s, so a 150 mm pipe is selected. [2]

Editorial calculation: at the same mass flow and pressure, required bore scales with the square root of the velocity ratio, so the 5,000 kg/h at 7 bar g example sized at the header's 15 m/s needs about 130 × √(25/15) ≈ 168 mm bore. [2] [1]

The header should be designed for the boiler operating pressure; Spirax Sarco notes that flange ratings fall with temperature, so a PN16 flange rated 16 bar at 120 °C is only suitable for up to 13.8 bar saturated steam (198 °C). [1]

Give the supplier the number of boilers and their outputs, design pressure and temperature, total and branch steam flows, number, sizes and positions of inlets and off-takes, drain and trap arrangement, isolation and non-return valves, pressure relief and instrument connections, supports, insulation, material and the design code and inspection required.

02

Compare header velocity limits between two boiler-house references

Boiler maker Fulton publishes the velocity basis its engineers use to size near-boiler steam piping, including the steam header, from the total horsepower of all boilers that can run at once and the operating pressure. It suggests 4,500 ft/min for near-boiler piping and explains what happens at higher velocities. [4]

Editorial calculation: Spirax Sarco's 15 m/s header limit above is about 2,950 ft/min, two-thirds of Fulton's 4,500 ft/min. For the same mass flow and pressure, bore scales with the square root of the velocity ratio, so a header sized at 15 m/s has about 1.23 times the bore of one sized at 22.9 m/s (√(22.86/15)); in Spirax Sarco's 5,000 kg/h at 7 bar g example, about 136 mm bore at Fulton's 4,500 ft/min against about 168 mm at 15 m/s. [4] [1] [2]

The two references size for different aims: Fulton links the upper velocities to water carry-over from the boiler water line, Spirax Sarco uses a low header velocity so entrained moisture can fall out to the drain. State which velocity basis the header supplier used, and give the combined output of every boiler that can run at the same time. [4] [1]

03

Lay out the header so boilers share load

Spirax Sarco explains that a “common header” fed from boilers along its length and discharging at one end lets the boilers nearest the off-take discharge more steam: it states that "typically, if boiler number 1 is fully loaded, number 2 will be around 1% overloaded, number 3 around 6%, and number 4 around 15% overloaded", which can make the water level unstable and lock the boiler out. It aims for pressure drops from each boiler outlet to the plant off-take within 0.1 bar. [1]

A header discharging from the centre keeps any boiler from being overloaded by more than 1% if the pipework is properly sized, and a branching "family tree" arrangement is recommended for four or more heavily loaded boilers with sequencing control. Where two or more boilers share a header, a second lockable stop valve is fitted in each boiler connection, and a non-return function is needed; simple flap non-return valves can oscillate and cyclically overload boilers. [1]

Spirax Sarco adds that a well-designed boiler under steady load produces steam with a dryness fraction of about 96–99%, and recommends a separator close to the boiler; the header gives an extra separating function if that separator is overwhelmed. [1]

04

Bring the header and mains on line slowly

Spirax Sarco warns that warm-up must avoid waterhammer, thermal shock and priming. A boiler main stop valve has a flat seat and passes about 80% of its capacity in the first 10% of its movement, so on larger plants a control valve after the stop valve, or a small control valve in a loop around an isolation valve, gives a controlled warm-up. [1]

For the mains leaving the header, Spirax Sarco advises a fall of at least 1:100 in the direction of flow (1:40 where lines rise), drain points every 30–50 m and at low points, and, "as a general rule", sizing branch lines of not more than 10 m on 25–40 m/s; where a main must rise against the flow, it suggests a velocity of no more than 15 m/s. [3]

05

One fabricator listing headers as sub-cylinders

Jiangsu Prettech is the company shown below; its page lists steam distribution headers, which it calls sub-cylinders.

The supplier reviewed builds headers (sub-cylinders) alongside tanks and heat exchangers; ask for the design code, calculations, material certificates and pressure test report.

06

Body, inlets, off-takes, valves, drain, relief and supports

Inlet positions belong on this list because, as the load-sharing section shows, where the boilers join the header decides how evenly they share load.

07

Bore, design pressure, outlets and material for the quotation

Boiler outputs, flows and off-take positions come from your plant. Spirax Sarco supplies the sizing rules in the middle column; shell thickness and material come from the pressure equipment code.

08

Send the boiler, flow and layout data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Boilers: [number; output; pressure]
Flows: [total; each off-take]
Design: [pressure; temperature; code]
Layout: [inlet and outlet positions; centre discharge]
Valves: [stop; lockable; non-return; branch]
Drainage: [drain points; trap type; discharge]
Documents: [calculations; material certificates; test report]

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