Knowledge for the Sulphuric Acid Industry
Sulphuric Acid on the Web
Acid Plant Database
Boiler Feed Water
Materials of Construction
DKL Engineering, Inc.
April 15, 2006
Gas-to-Gas Heat Exchangers
The life span for a piece of equipment varies with many factors such as:
Original design parameters
Materials of construction
Fabrication and erection
Operating conditions (normal and excursions)
Type and frequency of maintenance
The original design parameters for a piece of equipment play an important role in the expected life of a piece of equipment. The corrosion allowance and vessel wall thickness specified for a piece of equipment directly affects its life span. For lined equipment, the specification of the lining system and thickness is also an important factor. If a piece of equipment is subject to erosion then design velocities will affect equipment life. In corrosive services, the material of construction used to construct a piece of equipment affects its expected life span. Constructing a heat exchanger from stainless steel rather than carbon steel may cost more but its increased life span may justify the initial higher cost. The quality of the fabrication and erection for a piece of equipment is important in extending the life of a piece of equipment. Poor welds can result in premature failure or increased corrosion. All equipment is designed to operate within a set limit of temperature, pressure, concentration, velocity, etc. Operating outside of the design limits may result in increased corrosion or stress on the equipment that will lead to premature failure. No piece of equipment is expected to operate its entire life without some preventative maintenance. Vendors will often provide maintenance schedules and activities to be performed in order to maximize the life of a piece of equipment. When a piece of equipment has reached the end of its useful life it must be replaced. Equipment must be replaced when the cost to maintain it becomes so high that the cost of replacement can be justified. As well, if a piece of equipment is no longer reliable and results in frequent and costly outages, it should be replaced. Finally, complete failure of a piece of equipment such that it can no longer be operated means that replacement is required. A piece of equipment may also be replaced in conjunction with a plant expansion even though it still has years of serviceable life remaining.
The converter is the largest single piece of equipment in the plant. A decision to replace a converter is not taken lightly because it is a major undertaking. Converters that are being replaced at this time are the cast iron grid and post and the brick internal designs. There are no known cases of newer stainless steel converters being replaced. The older converters typically suffer from gas leaks, bulging shells, collapsed grids, internal/external gas leaks, etc. Since the converter is a major piece of equipment, a replacement converter is often accompanied with an increase in plant capacity and an increase in the size of the converter.
|Original 1961 cast iron grid and post converter replaced in 1999 with a new stainless steel design. Service life of 38 years. The converter was replaced as part of a plant increase in capacity from 2200 to 3200 STPD.|
|Original 1965 cast iron grid and post converter replaced in 2004 with a new stainless steel design. Service life of 39 years.|
|Sulphuric acid regeneration plant converter, originally built in 1972 was replaced in 1996 after a service life of 24 years.|
|Original 35 year old cast iron grid and post converter replaced in 2001 with a new stainless steel converter|
Phalaborwa, South Africa
|Plant built in 1975. Original converter replaced after a service life of 24 years with a stainless steel converter. Plant capacity also increased.|
|Original cast iron grid and post converter replaced in 2002 with a new stainless steel converter after a service life of 33 years.|
Trail, British Columbia
|Original converter in No. 8 acid plant was installed in 1966. Converter replaced in 2006 after a 40 year service life. The converter had undergone numerous repairs.|
A gas-to-gas heat exchanger reaches the end of its useful life when it can no longer perform the required heat transfer duty or when the pressure drop through the exchanger is excessive. The major reason for both of these conditions occurring is corrosion of the heat exchanger. Poor heat transfer can be caused by fouling of the heat exchange surface and corrosion of tubes the causes tube to be plugged. High pressure drops are the result of corrosion products accumulating inside the exchanger.
|Cold Exchanger||Typical service life: 5 to 15 years|
|Two 13 year-old exchangers replaced with Monplex units. Old units corroded, tubes plugged. High pressure drop low heat transfer|
El Teniente, Chile
|SO3 cooler originally installed in 1998. Exchanger re-tubed after 6 years in early 2004. Tube failures continue through 2004 and exchanger re-tubed at end of 2004.|
|Plant built in 1991. Cold reheat exchangers replaced in 2004 after a service life of 13 years. The hot and hot reheat exchangers were replaced in 2005 after a service life of 14 years. The replacement of the heat exchangers was part of a plant expansion. The cold reheat exchangers were showing high pressure drops which necessitated their replacement.|
|Cold heat exchanger replaced in 1997 after a 7 year service life. During previous shutdowns corrosion was noted at the lower inlet tubesheet and some tubes were plugged. Although the unit was still serviceable, a decision was made to replace the unit to ensure reliable operation.|
Phalaborwa, South Africa
|Plant built in 1975. Hot heat exchanger replaced after a service life of 24 years. Cold exchanger replaced after a service life of 26 years.|
|Replacement of heat exchanger E104 in 1995 after a service life of 17 years. Exchanger E104 is a hot exchanger in a sulphur burning plant.|
|Original hot heat exchanger installed in 1962 was replaced in 1971, 1980 and 1991 with a similar design (carbon steel with alonized tubes). In 2003, the hot exchanger was replaced with a stainless steel design.|
There are two types of distributors in common use: pipe and trough distributors. The material of construction is cast iron, high silicon stainless steel or 310 SS. The life of a distributor depends in the material of construction, acid temperature and concentration. For cast iron distributors longer service life is achieved by using thicker the walls for the pipe, trough and downcomers. Unfortunately, the thicker walls increase the weight and cost of the distributor. Alloy stainless steel distributors rely on the fact that corrosion rates are much lower so thinner material can be used in the fabrication of the distributor. Distributors are somewhat unique because they experience corrosion from both sides of the material whereas piping experiences corrosion only from the inside.
Cast Iron: Downcomers will typically last about 4 to 5 years
Alloy: Expected life of more than 20 years based on corrosion rates of less than 0.025 mm/y (1 mpy)
Properly design, installed and operated brick lined equipment should last the life of an acid plant. Properly specified and installed acid resistant lining will provide the necessary protection of the carbon steel shell. However, if there is a failure of the lining that allows acid to penetrate to the shell, the equipment will quickly begin to show the signs of deterioration. The first indication of a problem will be the characteristic bulging of the metal shell as iron sulphates form between the shell and the brick lining. The next stage will be actual acid leaks through the steel shell. Various techniques can be used to repair and extend the life of the equipment but eventually the equipment will need to be replaced.
40 year old drying tower replaced.
37 year old absorber tower replaced.
Uncle Sam, LA
|Absorber tower originally installed in 1968. The tower lasted 36 years but had been repaired numerous times. Approximately, 60 to 70% of the shell had been repaired or replaced.|
|Pasminco Zinc||Original Lurgi venturi absorber was built in 1978. Over the years the tower developed bulges, leaks and degradation of the carbon steel shell. In 1998, Lurgi replaced the tower with an increase in capacity (+15% flow). The tower had a life of 20 years.|
Memphis Site, TN
|Drying and absorbing towers were replaced as part of a plant upgrade to increase capacity by a factor of 2 times. The original 1972 drying tower was replaced in 1994 after a 22 year life. The original absorber tower was replaced in 2000 after a 28 year life.|
|CF Industries||Original 1965 drying tower was replaced in 1997 after a service life of 32 years. The original tower had a flat bottom design that was repaired numerous times. This was done in both the A and B plants.|
South Pierce, FL
Original 1975 absorber tower in the No. 10 plant was replaced in 1992 after a service life of 17 years.
Absorber tower in No. 11 plant was replaced in 1996 after a service life of 21 years.
|Original final absorption tower on the No. 5 plant built in 1982 replaced in 1989 and again in 1995. In 2002, tower was replaced with an alloy design.|
|Final absorption tower replaced in 2001 after a service life of 27 years.|
|Original absorber tower built in 1972 replaced in 1993 after 21 years of service. The replacement tower experienced problems early in its life and was eventually replaced in 2005 after only 12 years of service.|
Phalaborwa, South Africa
|Plant built in 1975. Original dry and absorber towers suffered from leaks and the integrity of the brick lining was in question. Both towers were replaced in 2001 with alloy towers after a 26 year life.|
Fort Madison, Iowa
|Plant built in 1977. Original drying tower and pump tank replaced in 2005 after a service life of 28 years. New drying tower is an alloy design.|
A sulphur furnace is basically a refractory lined carbon steel shell. The factors that most affect the life of a sulphur furnace are the operating temperature, thermal cycling and condensation of acid on the shell. Refractory lining failures are probably the most common problem associated with sulphur furnaces. Loss of refractory bricks from the lining will create hot spots on the shell causing damage to the shell. Collapsed baffle walls will change the flow pattern in the furnace and affect burning efficiency.
|After 18 years of operation the sulphur furnace was requiring frequent maintenance and repairs. The sulphur furnace and boiler were replaced and the plant capacity was increased at the same time.|
Prince George, BC
|Plant originally built in 1966. Sulphur furnace replace in 1989 as part of a plant expansion and addition of liquid SO2 plant. Service life of 23 years.|
|No. 8 acid plant originally built in 1966. Sulphur furnace replaced in 2003 after a service life of 37 years.|