Boiler Water Management Mountain Vista Medical Center

Introduction

Steam boilers are simply devices for converting water into steam, which is then used for comfort and process heating. Steam boilers are divided into various classes dependent upon the operating steam pressure; generally low pressure boilers operate at less than 15 psi, moderate pressure boilers between 15 and 100 psi, medium pressure units 100 to 300 psi, and high pressure units 300 psi to 600 psi. Boilers operated above 600 psi, up to 2500 psi, are generally power plant units.

Mechanically, the most common boiler design is the “fire tube”, where the combustion gases from burning gas, oil, coal, or wood are channeled through tubes which are surrounded by water. Mountain Vista Medical Center (MVMC) is equipped with two 300 hp fire tube boilers fired with gas and operating at 75 PSI. The boilers are alternated on a weekly basis and generated steam is used to heat domestic water, provide heat to the hot loops, and is the heating source for secondary steam generators for autoclaves used for instrument sterilization.

Scale

As boilers produce steam by heat transfer from hot combustion gases into water across a metal surface, control of scale formation, which substantially reduces heat transfer, is a very important consideration. The following table shows the loss of heat transfer efficient with increasing scale formation.

As you can see from the table, 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 that the reduction in heat transfer from the combustion gases the water results 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.

The thermal stresses induced by scale formation cause 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 also lead to replacement of boiler 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!

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. 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!

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. Softened water should be checked on a daily basis and remedial action taken if the total hardness exceeds 5 mg/l.

Scale control chemicals are then utilized to control the scale forming ions which pass through the softener, are present due to corrosion of the boiler system, and which could be introduced with return condensate form process or heat exchanger leakage.

The chemical scale control program, PCT 5302 M, designed for MVMC is based on sequestrant chemistry, phosphonate is utilized to sequester, chemically react with, scale forming ions in the boiler water, preventing them from forming scale on heat exchange surfaces. The sequestered hardness ions are then prevented from forming simple “mud” deposits by inclusion of a copolymer in the formulation. Scale forming ions are thus kept in suspension in the boiler water and removed in the routine blowdown.

Oxygen Corrosion

Most boiler systems are constructed of steel and are quite susceptible to internal corrosion by oxygen. Corrosion in a boiler system can occur in the deaerator, feedwater tank, all piping, boiler, 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. Historically, MVMC has experienced excessive corrosion rates in the feedwater tank and associated piping.

Oxygen is introduced through the feedwater and through the return condensate. Oxygen is removed mechanically by the use of a deaerator and chemically by the use of oxygen scavenging compounds. When there is not enough oxygen scavenger in the boiler water, the oxygen begins to attack the metal. Left under treated, these pits become holes. Once holes have formed, unlike scale, which can be removed, holes cannot be repaired, rather replacement is required.

The oxygen scavenger, PCT 5402, to be used in the ProChemTech water management program for MVMC is a USDA approved alkaline product, which will substantially reduce the historical corrosion problem. It is very important to maintain the specified 40 to 60 mg/l of sulfite in the boiler to prevent oxygen corrosion.

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 some cases, this can lead to steam pipe failure due to impingement erosion corrosion. Most steam users are designed for “clean” steam, not steam carrying impurities, and use of steam with impurities will often result in the user having difficulties.
The general school of thought to avoid foaming is to limit boiler water conductivity to a maximum of 6000 mmhos while keeping OH alkalinity below 850 mg/l as CaCO3.
Automatic blowdown control based on conductivity is highly recommended as a means to avoid foaming problems.

Most water treatment programs for boilers include an effective anti-foam compound, which is a component of the PCT 5302M used primarily for scale control. Effective active dosage is generally less than 0.05 mg/l in the boiler water.

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 condensate return piping, leading to premature and costly condensate piping failure. Corrosion in the condensate system is not only detrimental to the mechanical side of the boiler system, but also contributes to scale buildup in the boiler from iron transported in the condensate back to the boiler.

Hospitals generally have a complex steam and condensate system due to multiple uses and varying loads. Use of a multiple amine product is the best means to provide protection for all components of the boiler system and a dual active product, PCT 5720, has been specified. Condensate should be routinely tested at all sample points for pH, target range of 8.0 to 8.5, and product feed adjusted accordingly.

Boiler Chemicals

All boiler chemical programs are designed on the makeup water that will be used in the system. MVMC uses softened Mesa city water as boiler makeup; it has high dissolved solids content and is very alkaline. Cycles of concentration are limited by the makeup water alkalinity to prevent foaming at four (4). The following boiler chemicals will be used at MVMC.

PCT 5302 M is a USDA approved sequestrant, phosphonate and polymer, base boiler scale control product. This product is specifically formulated to tolerate hardness from softener overruns can be expected and where iron in the condensate can be a problem. As opposed to the “traditional” phosphate chemical program that forms “sludge” in the boiler as the means to prevent scale formation on heat exchange surfaces, sequestrant chemistry keeps any hardness and iron in solution and is removed via the blowdown. Due to interferences with the difficulty in testing for phosphonate, this product has molybdate as a tracer for easy control testing with control limits of 1.0 to 2.0 mg/l as Mo.

PCT 5402 is a USDA approved alkaline pH liquid sulfite product used as the oxygen scavenger. This product can be fed into the deaerator due to its alkaline pH. Control levels in the boiler water are 40 to 60 mg/l as sulfite.

PCT 5720 is a USDA approved blend of two neutralizing amines which are more effective than a single amine product in the complex steam and condensate return piping typical of a hospital. Product feed is controlled to obtain a condensate pH of 8.0 to 8.5 at all condensate sampling points.

Chemical Control and Feed System

The blowdown is currently automated using an AquaTrac MicroFlex controller activating an ASCO blowdown solenoid. The treatment chemicals are fed proportional to the feedwater used, which is not an ideal system from the control standpoint given the varying loads typical of a hospital. A makeup proportional system feeding directly from the shipping container is recommended for future consideration to eliminate routine handling of hazardous chemicals and the potential for incorrect mix downs.

Program Responsibilities

The ProChemTech service representative will routinely do two complete service calls per month as well as any additional calls to assist in resolution of any problems. The representative will undertake the prescribed boiler tests and based on the test results, will make the necessary adjustments and recommendations for proper chemical control of the boiler system. A written service report, detailing the test results, changes made, and any equipment problems noted will be prepared in either hard copy of electronic e-mail. Plant operations people will be interviewed and any problems discussed.

Plant operators are responsible to do the following boiler tests on a routine basis and record them on the monthly log sheet, noting any changes to equipment or feed rate.

Recommended Daily Boiler Testing

• makeup water hardness
• makeup water conductivity
• makeup water used (log)
• condensate pH
• boiler water molybdate
• boiler water sulfite
• boiler water conductivity
• boiler water OH alkalinity

Control limits are usage rates are summarized on the Chemical Treatment Specifications summary sheets following in section 2 of the Manual.

The monthly log sheet is also reviewed on a routine basis by the ProChemTech representative.

Allowing for our normal 5 working day chemical delivery, either the ProChemTech representative of a plant operator should place an order when there is approximately ¼ drum of product remaining. This is of particular importance considering that ProChemTech is a just in time manufacturer and product is made to order, always fresh! Product can be ordered via the WEB, orderentry@prochemtech, by phone to the corporate office, Barb at 814-265-0959, or locally to the Apache Junction plant, 480-983-5385, Robert or Riley, or by FAX, 814-265-1263.