In smaller systems it is common for one pump to be used for both pumping to the boiler and condensate return. Larger systems tend to require multiple pump banks that are dedicated to both processes.
When selecting pumps for boilers ensure you receive information on:
• Volume metric boiler feed water rate or Boiler MCR (maximum continuous rating which gives you flowrate)
• Pump discharge pressure,
• Inlet temperature of water, and
• NPSHA available
Method of control
There are two main methods of controlling a boiler.
1. Boiler on/off
The most cost effective way is an on/off control that utilises floats or electrodes mounted directly in the boiler and dependent upon the water level will start and stop the feed pump. Anti-siphon valves are also used on this control method which prevents the boiler from flooding. Boilers that utilize this type of control are of a smaller design up to 300kw.
2. Continuous operation
A water tube boiler comprises a number of tubes surrounded by fire created by gas or oil. The feed pump passes water through the tubes. The pump runs continuously whilst the flow varies depending upon the demand of the boiler. The pump will tend to need some form of protection to prevent running beyond its maximum capacity. Normally a bypass line is installed which is either left open to a tank or controlled via a modulating valve.
a. Modulation control
With this type of control the pump is controlled via a control valve in the main pump discharge line with a bypass. Anorifice plate is installed or a modulating valve fitted in the bypass line. The signal for modulation can be provided by either a level, pressure or temperature controller. The advantage of this type of control method is a smaller pump with better efficiency and reduced running costs.
When using modulating control it is essential to install a bypass pipe line back to the condensate tank or de-aerator. DO NOT RETURN TO PUMP SUCTION.
b. Bypass control
Another method of control is to place a modulating control valve in the bypass line; this acts in the opposite way to the valve above. As the water level in the boiler rises the modulating valve in the pump bypass line will open and return water back to the condensate tank thus reducing the water flow to the boiler. Normally a high-level alarm will activate if there is loss of boiler pressure.
In all boiler applications due to the use of the pump with higher water temperatures (often above 100°C), it is essential that the NPSH be calculated for the system as incorrect selection may result in the pump being damaged. It is not unusual for the height of the condensate tank or de-aerator to be set to ensure sufficient static head to provide the NPSHA.
Potential problem areas
Boiler manufacturers are acutely aware that their application can be extreme and that the pump may be operating constantly and at high temperatures. Problems are often attributed to the pump but when analysed, these problems are usually down to a system problem.
Typical problems are:
Cavitation: The pump or components are damaged and this occurs if the NPSH conditions are incorrect.
On/off control: The pump is working at the end of the curve at a bad NPSH point.
Modulating control: Can cause damage to the valve due to over-pressurization, as well as the pump itself.
Pipeline losses: If incorrectly calculated can increase the pressure the pump produces.
Making steam: Steam can escape back through the boiler check valve damaging internal pump components. ■
The Lowara e-SV Series pump range offer high temperature versions suitable for boiler feed applications up to 180°C. Contact your nearest Brown Brothers Engineers office today about your requirements www.brownbros.com.au