The issue of where to add phosphates to boiler water
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Dear Haihao friends, our company has two waste heat boilers with an operating pressure of 2.5 MPa. During the design phase, phosphates (both trisodium phosphate and disodium hydrogen phosphate) were added at the outlet of the boiler feed water pump; after passing through the boiler feed water heat exchanger, the water was heated to around 150 degrees before entering the steam drum. Recently, the automatic control valves for boiler feed water have been sticking frequently. Upon disassembly, scaling was found (the exact composition is unknown), but the results of our boiler water analysis are satisfactory (with basically no hardness). I’m not sure what could be causing this issue Is it okay for us to add phosphate in this way?
This post was last edited by zgj2405 on 2011-11-29 at 18:03. One of the main reasons lies in trisodium phosphate and disodium hydrogen phosphate; I’m unable to figure it out, and I hope experts on the chemical engineering forum can help.
It shouldn’t be that reason, as the amount of trisodium phosphate added is very small, far below its saturated solubility; we should look for other causes instead.
Personally, I think the concentration of the trisodium phosphate solution you added is too high. The component of the scale is likely trisodium phosphate that has precipitated.
This post was last edited by zgj2405 on 2011-10-8 at 14:02. It seems that the problem is mainly caused by incomplete dissolution of the chemicals; by modifying the chemical dosing device and connecting a chemical storage tank after the control valve, the issue mentioned by the original poster will not occur. For more details, please refer to the following thread: http://bbs.hcbbs.com/viewthread...0&page=1#pid5834937
It seems that the phosphate hasn’t dissolved properly, or there are impurities causing this issue
You are placing the phosphate solution in the wrong location; it should be injected directly into the drum using a metering pump (dosage pump). The drum is usually equipped with a phosphate dosing pipe, which has many small holes distributed along its length. The dosing pipe is positioned at approximately 100 mm below the normal water level of the drum.
Reply 1# sqh-02696: The safety valve of the metering pump used in Unit 1 may be leaking. 2. Impurities in the chemicals caused blockages in the pipes. 3. The chemicals were not added in the correct location; they should be added directly into the steam drum
Problems with the design done by the design institute. Adding chemicals at the outlet of the feed water pump has been a practice for decades; however, due to scaling in the pipelines and damage to the control valves, this method was later replaced by adding chemicals in the drum. We have typical cases for this issue. Phosphate scaling is quite severe; otherwise, it would be impossible to bind the ions in the boiler water.