7 replies to this topic

[Mayank Joshi]

In a Steam Drum phosphate is dosed for TDS control to remove hardness ,so if TDS of Steam is high we increase Phosphate dosing ? What action should be taken if Silica in Steam is high?

Also Continuous Blow Down is from the side of the Steam Drum and Intermittent Blowdown is from the Bottom of the reboiler .

 

So what are the Exact purpose of CBD and Intermittent Blowdown ??? 

 

also is there a thumb rule for CBD rate ? 
 

[PingPong]

There are a few things you should realise:

- If your BFW is of poor quality, then chemical dosing may not be sufficient to solve your problems,

- Purpose of continuous blowdown is to remove dissolved salts that may have entered the system via the BFW supply.

- CBD rate typically varies between 2 and 10 % of BFW supply rate, depending very much on the quality of BFW, and the pressure (temperature) of the steam drum.

- IBD is to flush out solids that may have accumulated at the bottom of the steam drum or other low points in the system.

[Mayank Joshi]

I am increasing Chemical dosing only if Steam TDS is high , not if Bfw quality is low , I was just wondering whether TSP removes hardness as well as Silica , or is silica removed by IBD , also since CBD removes sludge which floats on water it cannot be the same thing which is removed from the bottom of the reboiler since it is lighter that water and IBD material is heavier ..

[oscarsender]

This website may help you.

 

http://www.gewater.c...lerdeposits.jsp

[oscarsender]

Typical blowdown rate are as below:

 

Very high pressure system (69 barG and above): 1 – 2 %

High pressure system (40-69 barG): 5 – 20 %

 

It also depends on make-up water quality

Demineralized water:  1 % or less for very high pressure systems.

Softened water:  5 - 12%.

Filtered water: 10 - 20%.

[PingPong]

I am increasing Chemical dosing only if Steam TDS is high , not if Bfw quality is low , I was just wondering whether TSP removes hardness as well as Silica , or is silica removed by IBD , also since CBD removes sludge which floats on water it cannot be the same thing which is removed from the bottom of the reboiler since it is lighter that water and IBD material is heavier ..

If BFW quality is low, and your CBD is low, then the boiler water will be high in TDS and silica content. The TDS and silica in the steam increase with those in the boiler water, it is as simple as that, so if the steam TDS and/or silica are too high, your CBD is probably too low. There may be too much boiler water entraiment (carrying solids) in the steam drum, but there is not much you can do about that, except lowering the water level in the drum (but keep it above the CBD drawoff). Foaming may also cause water carryover into the steam. Foaming can be caused by organics in the BFW due to poor operation of the condensate polishing unit.

 

Dosing trisodium phosphate (TSP) will reduce dissolved Ca²⁺ and Mg²⁺, but I think it affects dissolved silica only slightly by its impact on the water pH, moreover it does not reduce the anions (sulphate et cetera) originally associated with the Ca²⁺ and Mg²⁺. It merely replaces dissolved Ca²⁺ and Mg²⁺ by dissolved Na⁺ so the TDS in the boiler water is not affected much. The precipitated calcium phosphate and any precipitated magnesium salts (phosphate, hydroxide, silicate) are to be removed every now and then by opening the IBD.

Edited by PingPong, 10 July 2014 - 05:02 AM.

[Mayank Joshi]

Pingpong you are absolutely right by saying that TSP does nothing to TDS it merely replaces the Cation , my question is that chemical dosing or blowdown in a plant are adjusted by checking the lab result ,egs for BFW we check hydrazine and if residual hydrazine is zero then we increase hydrazine dosing  , similarly Steam and blowdown sample are sent to Lab , what will happen if we increase frequency of IBD and will the same thing happen if CBD rate is increased ? How is Silica in Steam controlled as Steam sample is tested for silica also ?

[PingPong]

TDS and silica in the steam are related to the TDS and silica in the water of the steam drum.

Increased CBD will lower TDS and silica in the steam drum water, so TDS and silica in steam will drop too.

IBD is meant to remove solids that settled at the bottom, not to replace CBD.