can i get more details on this?

You have three questions, so let me answer these in sequence.

1. A bit of both I should say. If colloidal silica is present in FW, it would gradually increase the boiler water silica due to the COC (Cycle of Concentration) in the evaporator. The silica inclusion in the steam through the attemperation spray would be negligible as the spray quantity is considerably less as compared to the steam flow. The silica measurement also doesn’t have the level of accuracy to pick up that increase. But you would not be wrong if you worry about more silica reaching the Turbine if you are using high amount of spray to control the MS temperature, particularly if you suspect colloidal silica is present in your FW.

2. As dissolved silica is weakly ionized, it can be removed in SBA and not WBA. The MB ensures the removal of silica ions to less than 0.01 – 0.02 ppm. The slippage occurs due to improper regeneration of these equipments. Fouling of ion exchange resins is one of the major reasons.  It may sound odd, but the practice of regeneration of SAC on the basis of OBR may sometimes lead to silica slippage from SBA.

3. Ions like sulphate and chloride are pretty strong ions and therefore a WBA can remove these and thus reduce the ionic load for the SBA, which has a more difficult job of capturing the weak ions like nitrate, phosphate and particularly silica. One would use a WAC if the hardness of raw water is more.

  Rate this Comment     

There are basically three guidelines for water and steam quality control that are being followed by the industries. These are from CEGB, EPRI and ABMA. Then there is the International Association for the Properties of Water and Steam (IAPWS), who have developed water and steam quality standards for power station equipment of different types on the basis of theoretical studies and long-term experience in the operation of power plants in various countries.

The boiler water silica limits for various pressure systems are more or less similar in these guidelines. But if your OEM decides on a limit of 1.2 for your system, then you would be bound by that as this limit is not too far away from the international guidelines. After all, can you ensure zero mechanical carry over?

I have seen in some systems with less than 100 ksc pressure, they are operating blow down the moment drum silica exceeds 0.5 ppm which they are maintaining as limit. In such cases, the management may think of changing the practice to save unnecessary blow down loss.

Your second question is not quite clear to me. You open the CBD valve as required. IBD on the other should not be used unless you suspect sludge in your system. It is quick open and close operation and not for continuous operation as this would affect the circulation in the evaporator circuit.

  Rate this Comment     

I think we are getting confused here of the Non-reactive nature of Suspended and colloidal (non-ionized) with negative electrostatic charges at the surface of these particles. Dissolved solids are reactive because they are present as ions in the water.

Most suspended solids smaller than 0.1 mm found in waters carry negative electrostatic charges. Since the particles have similar negative electrical charges and electrical forces to keep the individual particles separate, the colloids stay in suspension as small particles. The magnitude of the zeta potential (Zp) is usually used to indicate colloidal particle stability. The higher the zeta potential, the greater are the repulsion forces between the colloidal particles and, therefore, the more stable is the colloidal suspension. To remove colloids, small particles have to be destabilized first and then they will form larger and heavier flocks which can be removed by conventional physical treatment. This is done by the surface positive hydroxides that are formed by the Alum reaction as mentioned by you.

  Rate this Comment     

I would rather try to reduce the un-reacted CaO in the fly ash, as this indicates excess and ineffective lime dozing. Are you using powder lime? You are required to use crushed lime with a top size of 1 mm. Obviously, you should also try to increase the SA: PA (by PA, I mean fluidizing air), by reducing the fluidizing air by a small marging and increasing the over bed air by the same margin. Changing the bed content by adding fresh bed material more frequently would not be a bad idea either, as Sulfation reaction is less effective when there is more ash in the bed content. You could actually be surprized with the resulting economics of this practice.

  Rate this Comment