BOILER - WATER SOFTENER

 

The recommended quality of Feed Water  to Boiler :

                        Hardness CaCO3       ---        5 max.

                        pH                               ---        8.5 to 9.5

                        O2                               ---        nil

                        Oil                                ---        nil

                        Total dissolved solids  ---        minimum possible to reduce blowdown

A conventional water softener incorporates the technique of counter flow regeneration. This technique is used to treat water having high hardness and bring it to acceptable limits for boiler.

The flow of water during normal operation and flow of brine (salt solution) during regeneration are opposite to each other. Using this method, treated water is produced with low residual hardness throughout the cycle.

Construction :

1. Softener vessel : This is made of FRP material and is fitted with plastic internal fittings. This has an opening at the top for connecting the multi-port valve. A plastic pipe is arranged vertically in the center with strainer each at inlet and outlet.

2. Resin : One full charge of cation exchange resin is filled in the vessel. At the bottom is coarse silex sand material, which supports the resin.

3. Multi-port valve : This is a single valve made of PVC material that facilitates water softening, regeneration and backwashing operations by letting the water or brine solution in or out of system. This is lever-operated and opens or closes respective ports as required eliminating the need of individual valves. It is suitable marked on the handle for various lever positions. 

The softener has 4 openings, one for raw water inlet, one for soft water outlet, one for brine suction during regeneration and one for drain. 

4. Brine tank : This is placed adjacent to softener vessel and is filled with salt solution. This tank is fitted with a float operated brine control valve for controlling flow of brine. 

5. Piping : The brine tank is connected to the multi-port valve by PVC pipe and has ejector near the valve. Ejector is used to suck a quantity of brine during regeneration process.

The multi-port valve also has a connection and piping for draining the vessel.

Normal operation : 

Filtered raw water enters from inlet port, travels down through center pipe and after passing from bottom strainer comes out to get in touch with the resin. Then it rises up through silex & resin and enters the top strainer, which further leads it to softener outlet. 

While passing through the resin, Sodium ions of resin are exchanged with calcium and magnesium to produce soft water. Following reaction takes place -

R – Na  + CaCO3  R – Ca  + Na2CO3 

R – Na  + MgCO3  R – Mg  + Na2CO3

Outlet water from softener will have hardness of less than 5 ppm. This procedure continues till resin is completely exhausted of Sodium ions. This is time when the softener requires regeneration.

During normal operation, water also enters brine tank and fills it to required level. Level is maintained automatically by float operated control valve. Thus, Sodium Chloride (NaCl) solution is prepared in the brine tank. The concentration of NaCl is 5 to 7 %.

Regeneration (Backwash) :  

The first step of regeneration is called Brine Injection wherein brine outlet port is opened and brine is allowed to come into softener vessel from ejector outlet. When required volume of brine solution is ejected out, float in the brine control valve drops to bottom and stops further brine suction.

This is followed by Slow Rinse process wherein the brine solution passes through the resin. During this, regeneration process takes place and Sodium ions are regenerated. Following reaction takes place -

R – Ca  + 2 NaCl  R – Na + CaCl2

R – Mg + 2 NaCl  R – Na + MgCl2

After regeneration process is over, Final Rinsing is carried out, wherein water flows into the softener from bottom and goes upwards through resin and out to drain. 

The total time taken for entire regeneration process varies with size but is usually maximum one hour.

Now the softener is ready for normal service.

IMPORTANT TERMS AND SELECTION OF SOFTENER :

1. Output Between Regenerations (OBR) : This is the total output of soft water that the softener can deliver before it is required to be regenerated. For a particular softener, OBR depends upon inlet hardness and total working hour. It is specified by the softener manufacturer.

Theoretically, 

OBR = (Resin quantity x Resin exchange capacity) / Inlet water hardness

2. How to select a softener :

1. Select the softener model based on flow rate required.

2. Decide OBR requirement based on maximum running hours of the boiler. Check the OBR capacity of the softener with respect to requirement.

3. If the selected model does not suit the OBR requirement, select the next higher model.

Example :

We have to select a softener for 2 tph boiler that operates for maximum 20 hours/day at a site where raw water harness is 100 ppm. Refer to the selection chart and technical data chart.

1. Since the maximum flow rate required is 2 m3/hr, softener model 3A is selected.

2. OBR required will be 2 m3/hr x 20 hrs = 40 m3.

3. From the chart, it can be seen that model 3A has OBR capacity of only 28 m3, hence select next higher model 3B, which has OBR capacity of 55 m3.

3. Double Stream Softeners :

Double stream softeners are preferred for large capacity & continuous working boiler in case of very high hardness in raw water. 

This scheme consists of two softeners in parallel, one working & the other stand by. 

PHILOSOPHY :- In two stream system, when one softener runs in normal mode, the other softener will go for regeneration. This will ensure continuous supply of soft water to boiler.

DESIGN :- In this system, two designs are possible –

(a) Both the softeners can be designed for 12 hours cycle or 

(b) One softener designed for 20 hours cycle & other is for 4 hours cycle (For regeneration period only) 

The second scheme is especially more beneficial in case of high hardness in raw water.