Importance of Maintaining Proper Chemical Treatment of Boiler Water

For those of you who aren’t familiar with me, my name is Vickie Jacklin, Corporate Service Manager, for ProChemTech International, Inc.  I’ve been with PCT for 18 years.  Our company provides the chemicals and service to treat your boiler.  Paris uses a Hearst, gas fired, fire tube boiler for steam production in the industrial laundry process.  I’ve been asked by Tommy Walsh to explain the importance of maintaining proper chemical treatment in the boiler water.

System Overview

Right now, Paris has one gas fired Hearst boiler used for steam production.  Steam is used to heat the water in the tunnel washers.  On cooling, the steam condenses to   condensate, most of which is captured and returned to the boiler feedwater tank. This tank is float controlled and as more water is needed to replace lost steam, softened city water is added.  When possible, it is a good idea to have a steam sparge in the feedwater tank to keep it hot,200 to 212, which minimizes use of the chemical oxygen scavenger.  When the boiler calls for makeup water, via the McDonnell Miller valve, it is pumped from the feedwater tank.  Treatment chemicals are injected into the feedwater tank.

Chemicals are added to the feedwater tank for two reasons:  protection of the feedwater tank from rapid corrosion, since it is commonly constructed of steel, and to get proper mix (reaction) time prior to the addition to the boiler.

The addition of treatment chemicals to the boiler system, and boiler blowdown, is required to prevent formation of scale on heat transfer surfaces, deposition on heat transfer surfaces, boiler foaming, and to control corrosion of the boiler and steam system.

As with all water treatment programs, good control of the specific program selected is the major criteria for obtaining good results.  The primary function of boiler water chemicals is to “deal” with the “impurities” introduced into the boiler via the makeup water.

Scale

Scale in boilers in normally caused by precipitation of calcium, magnesium, silicon, and iron compounds on heat transfer surfaces.  Calcium, magnesium, and silicon are usually introduced with the makeup water, while iron is commonly generated within the boiler system by corrosion of the condensate system piping.

The biggest problem that scale causes in boilers is loss of thermal efficiency; basically, you are paying to heat the great outdoors.  The following table relates scale thickness to loss of heat transfer efficiency:

As you can see from the table above, the scale thickness may appear slight, but the efficiency lost is actually quite large. On a dollar and cents basis, the fuel cost for operating a 100 hp boiler with no scale is $602/day with gas at $6.00/1000 cut ft. An efficiency loss of 10%, less than 1/16 inch of scale, increases that cost by $60/day.

A second big problem with scale is the reduction in heat transfer from the fire to the water, resulting in the boiler fire side running hotter than designed.  Operating above design temperature places considerable thermal stress on the boiler internals and often causes warpage, water leaks, tube failures, and boiler refractory cracking/failure.

Scale formation causes the tubes to expand and contract during firing which will eventually lead to leaking on the tube sheet.  Because the tube ends are rolled into the tube sheet, these rolled ends are highly susceptible to premature failure.   This in turn will lead to the necessity of re-rolling the tubes.  There are only so many times this can be done before the tube is weakened and must be replaced.  The excessive heat and water leaks in turn leads to replacement of the refractory.

A quick indicator of scale buildup is the boiler exhaust stack temperature; it should be 100 F to 150 F higher than the boiler steam temperature. For a 100 psi boiler, steam temperature is 328 F so the stack temperature should be in the range of 428 to 478 F. If it is over 500 F, you have a scale problem!

The first line of control is to properly treat the makeup water for removal of the offending ions.  This is most generally done by softening the makeup water.  Properly maintaining the softener not only minimizes the chance of scale formation on the tubes; but also, allows the boiler chemical treatment program to be economically applied to control what scale forming ions do get through the softener.  Therefore, it is critical to the operation of the boiler that the softener be maintained in proper working order at all times. Although scale can be removed, it is quite costly to do so not only in man hours but also in boiler wear and tear.  For those of you who are not familiar with cleaning a scaled boiler, the first time you experience it, you will only hope it is your last.  Boilers are designed with the thought in mind that the tubes will not be scaled; therefore, scale removal is NOT easy or convenient!  Try putting your arm inside a hand hole, filled with cold water and scale, laying on your side, in a cold room, trying to pull as much scale out of this little hand hole as you can.  Did I mention the room was cold, the water was cold and oh yes, the scale is quite abrasive on your skin?  It doesn’t make for a fun day!

Foaming

Another common problem is carry over of boiler water with the steam, commonly known as foaming or priming.  When foaming occurs, any impurities in the boiler water are carried out with the steam and thus end up in the process.  In the case of Paris, this could equate to “stained” laundry.  The general school of thought is not too exceed a conductivity in the boiler of 6000 mmhos.  Even though you have an automatic blowdown on your boiler system and your present chemical program does not form a “sludge” in the boiler, it is still good practice to perform a routine manual blowdown on the boiler.

Condensate Corrosion

When steam condenses, it forms carbonic acid.  If not properly treated with either a filming or neutralizing amine, the acid will corrode the steam system, leading to premature and costly condensate piping failure.  Corrosion in an industrial laundry is not only detrimental to the mechanical side of the application but also to the laundry itself.  Iron in the wash water will react with specific laundry additives and cause spotting. This will more than likely result in load rejection thus higher laundry costs.  This is a good example of what happens when you haven’t been performing your routine boiler tests.  On the production line, spotting is found on the laundry.  Unfortunately, valuable time and money may be lost evaluating the laundry additives when in fact in may be from the condensate system.

Oxygen Attack

Corrosion in a boiler system can occur in the preboiler equipment, steam lines, and condensate return system.  Corrosion, by itself, is bad in that it can speedily destroy expensive to replace equipment with costly downtime from loss of steam for production use.  Even though at some point Paris plans on installing a second boiler, it is still vitally important that not only each boiler be properly maintained but also remember, each boiler will utilize the same feedwater tank and condensate system.

Most boiler systems are constructed of steel and are quite susceptible to internal corrosion by oxygen.  Oxygen is introduced through the feedwater and sometimes,

through the return condensate.  Oxygen is removed mechanically (by the use of a deaerator) and chemically (by the use of oxygen scavenging compounds.)  In the case of Paris, it is removed chemically as there is no deaerator in place.  When there is not enough oxygen scavenger in the boiler water, the oxygen begins to attack (otherwise known as pitting) the metal.  Left under treated, these pits become holes.  Once holes have formed, unlike scale, which can be removed, although not without difficulty, holes can not be repaired, rather the entire tube must be replaced.

So far, we have discussed what happens when you don’t properly treat and maintain your boiler system.  Now, we’ll talk a little bit about the chemical program and chemical feed system we are using here at Paris.

Chemistry / Chemical Feed Equipment

All chemical programs are designed based on the makeup water that will be used in the system.  DuBois city water is fairly soft; however, as makeup water for a boiler, the maximum allowable amount of hardness is 5 ppm (0.29 gpm.)  DuBois city water hardness varies throughout the year depending on the supply wells used.  Therefore, it is common practice to use a softener on boiler systems in the DuBois area.  We also have to be very careful of the amine used in the condensate system here at Paris.  While this is not generally a problem elsewhere, one amine in particular (cyclohexylamine) can not be used.  Based on the above criteria, the following chemical program has been selected:

PCT 5407 is a liquid sulfite solution used as the oxygen scavenger.  We have also added some polymer in the formula for deposition control.

PCT 5305B is the primary sequestrant based boiler scale control product.  This product is specifically formulated for systems where softener overruns can be expected and iron in the condensate can be a problem.  As opposed to the phosphate chemical program that forms“sludge” in the boiler, phosphonate keeps any hardness and iron in solution and is removed via the blowdown.  Due to interferences with city water phosphate and the difficulty in testing for phosphonate,  this product has been formulated with Boron for easy control testing.

PCT 5715 is 20% morpholine amine used to neutralize the condensate.

Chemical Feed System

The chemical feed and blowdown have been automated using an Advantage SS controller, a contacting water meter, chemical metering pumps, and an ASCO blowdown solenoid.  The chemicals are fed proportional to the makeup water used and the blowdown is controlled by the conductivity.

1. Automation of the system compensates for changes in steaming and condensate return rates.
2. Use of direct chemical feed from shipping containers eliminating routine handling of chemicals and dosage miscalculations.
3. Blowdown is optimized thereby minimizing water and chemical usage.

Job Responsibilities

As your service representative, it is my responsibility to come in on a routine basis, typically once per month and undertake the prescribed boiler tests.  Based on those test results, I will make the necessary adjustments.  I will leave a copy of the service report, detailing the results, changes made, and any equipment problems noted.  If there are major problems or concerns, I will discuss those with the appropriate Paris personnel.

While you may, at times, consider me a “nag”, if there are major problems, it is my responsibility to make sure they are corrected.

Your responsibilities are to perform the same boiler tests on a routine basis and record them in your log book, noting any changes to equipment or feed rate.  The test results are also emailed to me on a weekly basis.  If there are areas of concern, I will email back my recommendations.

Allowing for our normal 7 – 10 day chemical delivery, it is recommended that you place an order when there is approximately ¼ product left in your drum.  This is of particular importance considering we do not warehouse our chemicals.  They are made when the order is received guaranteeing fresh product to the customer.

As you can see from the above items, what might seem insignificant really does have a snowball affect.  In short, when either one of us neglects our responsibilities, production and maintenance costs increase, overall profits for Paris are lower, and in the end, that means less income for you, the employee and the loss of an account and income for ProChemTech.  So in short, Paris and ProChemTech will both operate more efficiently and profitably if we all do our part and work together.