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High Sulfite Residual

Sunday, December 31, 2017   By: Freeman [2215] 0 Stars
Hi all,
We have two type boiler in our gas processing company, with the following condition:

1.High Pressure Boiler P36bar T320 C
Blowdown Water: pH10.5-11.5 TDS 500ppm PO430-50 ppm SO320-40 ppm

2. Waste Heat Recovery Boiler WHRB:P20 bar T214 C
Blowdown Water: pH10.5-11.5 TDS 300ppm PO430-60 ppm SO380-100 ppm

Sulfite is injetcted in DA reservoir tank. Although, the feed water of both boilers are the same ph8.3 TDS2ppm and cycle concentration in WHRB is lower than HP boiler but sulfite residual in WHRB are higher than High Pressure Boiler.

We re going to find out the reason of high sulfite residual in WHRB, could someone explain whats the reason of this phenomena? The standard limit for sulfite residual is 20-40 pmm, Is this high level sulfite residual in WHRB harmful?

Thanks in advance!

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Wednesday, January 03, 2018   By: Ross. [390] 5 Stars
First, I want to say that large amounts of Sodium Sulfite NaSO3 becomes a contaminate because it will produce foaming and other problems, but foaming is the primary problem.

Your 500-psi Water Tube Boiler is seems to have reasonable level of residual Sodium Sulfite NaSO3 remaining in the boiler water. Nice pH too.

Your 300-psi Waste Heat Recovery Boiler has twice what I would expect for Sodium Sulfite NaSO3 at ranging 80-ppm to 100-ppm. pH is good so you are getting a good yield.

As you probably know Sodium Sulfite NaSO3 does not decompose until around 1800-psi.

So, my thoughts are the 500-psi Water Tube Boiler has some troublesome contaminates that are consuming the Sodium Sulfite NaSO3. But I am assuming you are using the same Deaerator, FW Pumps and piping.

Hope this helps and Good Luck. Ross

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Wednesday, January 10, 2018   By: Freeman [2215] 0 Stars
Thanks for your helpful reply.

We have been investigating the reasons of this issue.

For boilers, the deaerator is the same but pumps are different. Steam turbine pumps are used for HP boiler and electrical driven pumps are used for WHRB. After investigation we see that after putting in service one more Steam turbine pump for HP boilers, sulfite residual increase again in WHRB.

Is that possible that air leakage happens in steam turbine pumps and consume sulfite ?

Thanks in advance for your helpful guide!

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Wednesday, January 10, 2018   By: Ross [390] 5 Stars
Maybe you could comment on the piping on the deaerator to the Feed Water Pumps. Does the FW Piping to the Steam Pumps draw DA water from a different outlet in the DA than the Waste Heat Recovery Boiler?


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Wednesday, January 10, 2018   By: Freeman [2215] 0 Stars
After DA, theres main header which suctions of steam pump and electrical pump are connected to it.

We measure the sulfite residual in HP and WHRB boilers twice a day and change the sulfite dosing pump stroke based on HP boiler result, we try to keep sulfite residula in HP boiler in range 20-30 ppm.
Recently after putting one more steam pump, we had to increase dosing pump stroke for keeping sulfite residula in HP boiler in range 20-30.

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Wednesday, January 10, 2018   By: Ross [390] 5 Stars
You may want to consider the waterside deposits in the 500-psi High Pressure Watertube Boiler is consuming the NaSO3.

You have a common NaSO3 injection pump into the DA Tank. All FW Pumps have common Inlet piping. The differences are the pumps, which seem reasonable to rule out. Then we are left with the discharge piping and boilers are the difference.

It is not a horrific spot to be in and maybe consider a more comprehensive waterside deposit investigation including the discharge piping wet surfaces.


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Wednesday, January 10, 2018   By: Jim Watts [764] 5 Stars
It now seems clear that the high sulfite in the WHB is caused by increasing dosing to suit extra demand on the High pressure Boiler.
With the coincidence of demand increasing with BFP pump operations the likely spotis the pumps.
Air cannot get into the pumps directly but there is a path in via gland sealing water.
Check the source of the gland cooling water and figure out a way to make sure no Air is being injected.

BTW I assume you do not have a spray attemperator as solids should not be injected at the DEA in such a case.

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Sunday, January 07, 2018   By: Jim Watts [764] 5 Stars
The most likely cause for having too much Sulfite is that you are pumping too much in.
However the numbers you present are inconsistent and difficult to diagnose confidently.
Your High pressure has the lowest reported chemicals but the highest TDS. This is probably because you are not neutralising the sample to remove OH alkalinity before measuring conductivity. as OH has 3 times the conductivity of other components the actual TDS may be lower than reported. You do not indicate the actual PH and each boiler could be at opposite ends of the range reported.
You sulfite units appear to be the result of different procedure as one is reported as SO320 and one as SO380 and who knows what mysteries that represents.
It is not clear if there is one DEA or two involved but it is hard to see two different tests required if only one is used.
Sulfite decomposition would be different at the different pressures and my be contributing to some of the issue.
It would make sense to me to reduce sulphite until you got nearer to the minimum on the HP boiler if they are in fact common feed and obvious if they are not.
Running high levels is a waste of money at minimum and tending to problems you dont need.
It is also normal to run the TDS in the high pressure boiler lower than the WHRB.

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