Steam boiler water level controls and alarms an overview
Steam boiler water leve This brochure gives an introduction to modern methods of steam boiler water level controls and alarms, and outlines some of the systems available from Spirax Sarco. Significant developments in recent years have led to improved safety and reliability of boiler level controls, and Spirax Sarco is at the forefront of this technology. Electronic level controls are now standard, and have largely replaced older high maintenance float based devices. More detailed technical information is given in separate literature.Your local Spirax Sarco Engineer will be pleased to advise on the most suitable system for your application.
User benefits • Accurate safe control • Choice of systems for all applications • Approved by many authorities around the world • All systems suitable for pressures up to 32 bar g • On/ off or modulating control to suit application • Single, two, or three element control
Why do I need boiler water controls and alarms? Controls To achieve a steam output matched to the varying requirements of the average steam plant, good control of boiler water level is necessary. With the small steam spaces now common in modern boilers, a quick and accurate response to variations in water level is essential.
Alarms (limiters) In order to operate safely, all steam boilers need a method of warning of excessively low or high water levels. In many countries, regulations require two independent low water level alarms, or limiters. Independent high alarms are also required in many cases. With the move towards unmanned boiler houses, there is an increasing need for high integrity self-monitoring alarms. These systems can operate with extended test intervals in many cases.
What methods are used to control the water level? Boiler water level is controlled either by switching the boiler feedpump on and off at water levels determined by a probe and controller (on /off control), or by progressively opening or closing a feedwater valve as the steam demand causes the water level to change in the boiler (modulating control). On /off control On/off control systems are most suitable for installations where a certain amount of variation in boiler pressure and steam flowrate can be tolerated. Examples might be smaller boilers with stable load conditions.
On /off control
Steam flowrate
Modulating control Modulating control gives a steady pressure and steam flowrate in steam boilers. The feedwater flowrate is varied, for example through a modulating valve, in response to changes in water level. With modulating control, the boiler feedpump is running all the time, and a spill-back line used to return unused feedwater to the feedtank. Steam flowrate
Modulating control
Water level up and down
Fuel
Feedwater on / off
Feedwater valve Firing rate varies
Feedpump
Fuel
Feedwater flowrate matches steam demand
Steady firing rate
3P 15 M
Spillback line
2P M 14
M 4P 16
1 PM 13
Steam flowrate from a boiler with modulating level controls
1 PM 13
3P 15 M
Feedpump 5 PM 17
2P M 14
2
Steam flowrate from a boiler with on / off level controls
Water level more steady
M 4P 16
5 PM 17
el controls and alarms The majority of boiler level controls and level alarms operate on the principle of conductivity or capacitance, and Spirax Sarco is the world leader in using these principles for safe accurate control of water levels in boilers.
Conductivity or Capacitance? Conductivity A conductivity probe (LP10-4) and controller can be used in a boiler or a tank for on/off pump control and /or alarm duties. It has one electrode for each function, which is cut to the required length on installation. Each probe electrode acts as a simple switch, indicating a low resistance to earth if in water, or a high resistance if out of water. For example, with a low alarm, the controller senses a change of resistance from low to high as the water level falls below the probe tip. This change in resistance causes the controller to disengage a relay to operate an alarm bell, light, or both, and normally will also cut the power to the burner. Simple conductivity probes operate on this principle, and have no self-checking facility. High integrity self-monitoring level alarms (limiters) (LP30 low alarm and LP31 high alarm probes) operate on this same conductivity principle. The probes are designed to detect numerous failure modes and will, for example, sound an alarm if the wiring is damaged. The controller has a cyclic selftest facility which sounds an alarm if it fails to react to a simulated fault. Boilers fitted with alarms of this type in the boiler shell can operate with extended test intervals in most countries, in many cases allowing the boiler house to be de-manned.
On / off level control and simple high and low alarms. An additional low alarm would be needed for a boiler. (Protection tube not shown).
Used as an on /off adjustable level control, it avoids having to cut probe tips to length, and permits level adjustments to be made at the controller without having to remove the probe from the boiler.
LP20 / PA20 capacitance probe Providing modulating level control plus one of the alarms in a boiler. Additional alarm(s) would be needed.
Protection tube
Transmitter 100 %
Adjustable high
High alarm
or low level alarm
Valve closed Adjustable proportional band
Adjustable set point
Pump off Water level
Low alarm level
A high or a low level alarm switching point can also be selected. Controllers are available for pneumatic or electrically actuated valves or variable speed pumps.
Boiler shell
LP10-4 four-tip conductivity probe-
Pump on
Capacitance A capacitance probe (LP20 with PA20 preamplifier) can be used in a boiler or a tank for modulating or adjustable on /off control. The probe consists of a conductive core completely sheathed in an insulating material. For this reason, capacitance probes cannot be cut to size, so are available in several different lengths. As the water level rises or falls, the capacitance coupling effect changes.
Comparator tip
Valve open Transmitter 0 %
LP30 low alarm probe /limiterConductivity probe with comparator tip providing a high-integrity, self-monitoring low level alarm in a boiler.
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How do I chose the best system for my particular application? Two system examples are shown below. Many other combinations may be used to comply with local /national regulations and specific customer applications. Additional literature is available which gives detailed technical information on each system and its components. Example 1: On /off level control with simple low and high level alarms This system is suitable for a smaller boiler, or one where there is not a great variation in steam load, and is a common arrangement for supervised boiler houses where daily testing is carried out. The probes may be mounted in side chambers, as shown, or in the boiler shell. The probe tips are cut to length on installation to give the required switching levels. Fig. 1
LP10 - 4
LP10 - 4
Example 2: Modulating level control with high integrity self-monitoring alarms Typically used for unmanned boilers where the steam load varies significantly. Extended alarm testing intervals are permitted in many cases. LP20 / PA20 LP30
LP30
Fig. 2
Protection tubes
High alarm 1st low alarm
Pump off Pump on
Boiler High alarm Valve closed Valve open
2nd low alarm
1st Low
2nd Low Two LP10-4 probes used for level control and alarms in a boiler. Probes can also be mounted directly in the boiler shell in protection tubes. (Unused probe tips not shown).
Boiler shell mounted LP20 / PA20 used for modulating level control and high alarm. Two LP30 probes are used for first and second low alarms.
On/off level control with high integrity self-monitoring low level alarms (limiters) In many countries, high integrity self-monitoring low level alarms mounted in the boiler shell allow testing intervals to be extended. The boiler may then be operated without constant professional supervision. Modulating level control with simple alarms A capacitance probe is used for modulating level control and first low alarm, with second low and high alarms provided by a conductivity probe. Modulating control is able to follow variations in steam load more closely than on/off, minimising cycling of the steam output. This is a common arrangement for supervised boiler houses where daily testing is carried out. Two element control The signal from a steam meter is used to adjust the signal from a level controller and move the set point. This enables the system to cope with the large sudden variations in steam load typically found in laundries or breweries. Three element control Useful for heavily loaded multiple boiler systems and large water tube boilers on a feedwater ring main. Similar to two element control, but also uses the input from a water flowmeter in the feedwater line. Some of the products may not be available in certain markets. Spirax-Sarco Limited, Charlton House, Cheltenham, Gloucestershire, GL53 8ER UK. Tel: +44 (0)1242 521361 Fax: +44 (0)1242 573342 E-mail:
[email protected] Internet: www.SpiraxSarco.com © Copyright 2004
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Spirax Sarco is a registered trademark of Spirax-Sarco Limited
SB-P402-101
AB Issue 4
