Why Do Waste Heat Boilers Fail?: Excessive Temperature

SRU WASTE HEAT BOILER FAILURES

Sulphur 354

 | September - October 2014

 

www.sulphurmagazine.com

 

1

W

hy do waste heat boilers in sul-

phur plants fail? We can look to

the auto racing industry to find the

answer. Engine size in Formula 1 race cars

has continually reduced over the years as

engineers have found ways to make the cars

go faster and faster with a given engine size.

On its super speedways, NASCAR places

restrictor plates below the carburettor to limit

engine output. Similar efforts take place in

almost all types of racing. Why? Because

the limiting factor is the ability of the driver

to react quickly enough to safely control the

car. Waste heat boilers fail because we try to

operate them at levels beyond which we can

adequately design and safely control them to

provide reliable operation.

Typically waste heat boilers (WHBs) fail

due to three factors: excessive tempera-

ture, excessive mass flux rate and exces-

sive water-side fouling

1

. Similar to the racing

industry, the sulphur industry has increased

temperatures and mass flux (process flow)

rates to obtain greater unit capacity. How-

ever, this push has exceeded reasonable

bounds, to the extent that reliability of a

unit can be and has been compromised. To

maintain acceptable discharge environmen-

tal criteria, often the sulphur recovery units

(SRUs) must operate with significant vari-

ance in acid gas flow rates –

variances that

are not controllable by the SRU operators.

Water-side fouling is potentially affected by

these same parameters and becomes a sig-

nificant factor for

reduced reliability

.

Excessive temperature

Reaction furnaces (e.g., in refinery service)

were originally operated with a

combinatio

n

of amine acid gas and sour water acid gas

burning in a sub-stoichiometric combus-

tion environment using atmospheric air

as the oxygen source. As plants were de-

bottlenecked for needed increased sulphur

capacity, it was determined that increas-

ing the oxygen content to the burner would

allow increased capacity with the same

furnace and boiler. Improved burner tech-

nology allowed even greater increases in

oxygen enrichment and, consequently, more

sulphur was produced by the same plant.

This added oxygen technology has become

a standard offering for new SRU units.

However, the higher temperatures encoun-

tered with oxygen enriched operations has

resulted in cases where we are bumping up

against the sulphur plant’s “speed limit”.

The continuous operating temperature

limit for modern, well-designed and installed

thermal protection systems (both refrac-

tory and ferrule systems) is approximately

1,540°C. A well designed system (please

note the emphasis on the word system)

can operate successfully for brief periods

of time at temperatures above 1,540°C,

but only by sacrificing reliability. If a

3-4 year

life is desired for the thermal protection

systems, the design operating t

emperature

should be somewhat less than 1,540°C. A

100°C buffer between the normal and maxi

-

mum operating temperature is about the

minimum that can be used to protect the

thermal protection system from negative

(i.e., >1,540°C) conditions. With this small

margin for error, it takes a well-designed

and calibrated temperature measurement

system, a process control system, and

vigilant operators to control the temperature

and avoid reducing the reliability of these

thermal protection systems. Increased tem-

peratures affect the reliability of the WHB

by increasing the temperature of the metal

parts, which can increase corrosion. It

also

increases the heat flux through the tubes,

which can result in a Leidenfrost steam blan-

keting condition that usually occurs at the

end of the ferrules

2,3

.

The ability to accurately simulate and

monitor the furnace core gas temperatures

is often a problem that can result in exces-

sive operating temperatures. It is common to

use both pyrometers and thermocouples to

provide the best possible temperature meas-

urements. However, during shutdown inspec-

tions the refractory and ferrule materials

will often indicate that the operational tem-

peratures were actually above 1,650°C. At

the same time, the process control system

temperature measurement historical data

often does not indicate temperatures above

1,540°C and sometimes not above 1,425°C.

One strategy for high temperature shut-

down protection is to use thermocouples

in the furnace with the shutdown set at

1,540°C for five minutes. The thermocou-

ples, which measure the hot face of the

refractory in the furnace, indicate a lower

temperature than the core gas tempera-

ture entering the WHB. This differential

temperature is typically 110°C or more.

It should be noted that pyrometer tem-

perature measurements may be highly

influenced by the process gas analysis

change (such as occurs with oxygen enrich-

ment). Pyrometers set for air only will

normally read low by as much as several

hundred degrees. As a caution, we would

suggest that there are inherent inaccu-

racies of all temperature measurement

devices due to installation, location, cali-

bration, interference, maintenance, etc.,

issues. Therefore, any specific plant read-

ing can be off by as much several hundred

degrees C. This discrepancy is normally

lower than the actual temperature.

Hot standby operations have the poten-

tial to produce excessive temperatures;

therefore, tempering of the sub-stoichio-

metric hydrocarbon or hydrogen combus-

tion is necessary 

1

. Short term hot

standby

operation of much less than an hour with-

Why do waste heat

boilers fail?

D. Martens

 and

M. Porter

 of Porter McGuffie, Inc. and

L. Stern

 of Stern Treating & Sulphur

Recovery Consulting, Inc. examine the main causes of

waste heat boiler failures in Claus

sulphur recovery units and discuss what lessons can be

learned from past WHB failures.