Heat exchanger leakage cause investigation

Heat exchanger leakage cause investigation

Shell and tube heat exchanger is petrochemical profession widely used heat exchange equipment, its quality directly affects the petrochemical enterprise’s safety and economic benefits.
Now, because of various elements of the onset of heat exchanger leakage, constitutes the material waste and environmental pollution, affects the normal operation of production equipment and the economic benefit of the factory, has become a yield not ignored in question.
Know the region and the attack of heat exchanger leakage may attack leakage throughout elements, and from the planning, production, using the following three aspects of control, rate of leakage of heat exchanger can be greatly reduced.
The author of many different varieties of heat exchanger area and the leakage of seizures leaked the elements are analyzed, and put forward corresponding countermeasures, for heat exchanger’s planning, production and supply refer to use.
Primary 1 heat exchanger leakage mechanism 1.1 temperature stress and thermal fatigue

Heat exchanger leakage cause investigation

Heat exchangers are widely used in the petrochemical profession, steam attack, reboiler, such as boiler feedwater preheater are typical heat exchange equipment.
Most of the heat exchanger in the process of operation, the tube must exist between the shell side fluid temperature difference and tube side heat exchanger in many process there is a larger temperature difference between heat exchange tube, temperature difference between left on both sides of the tube plate and the heat exchange tube onset temperature stress.
When the temperature stress to must value, metal plastic deformation and creep will attack.
If the operating temperature of the heat exchanger changes periodically, or operating conditions for repeated the process of pressure, temperature and pressure relief, cooling, so repeated thermal stress change can make the equipment heat exhaustion.
Query, found that many refinery steam attack because of the elements and let the cat out of the failure.
1.2 control oscillation

Medium is perpendicular to the regulation of lateral activity is the first source of fluid induced vibration, if gas is flowing into the shell side, in a direction perpendicular to the activity direction and the pipe axis will constitute acoustic standing wave.
As the frequency of the acoustic standing wave and karman vortex frequency and turbulent buffeting, the main frequency, acoustic standing wave oscillation will provoke attacks.
In the process of operation and control oscillation will cause between the tube plate and pipe weld metal porosity and other defects existing in the development or development, development to must degree of these shortcomings will be breakdown or causing fatigue cracking.
Oscillations associated with the natural frequency of the heat exchanger tube and the inherent frequency is associated with control of the structure, scale, thus the heat exchanger’s own planning elements of a defect is the control oscillation.
The query analysis of many sets of reboiler [2, 3], and high pressure steam in the shell side walk, walk liquid passes, are leaked from operation shortly after the attack, and the revised rules to every three months.
Thinking about the temperature difference stress at first, but from the beginning planning production added reboiler swell, situation is still not improved.
And yangzi petrochemical aromatics plant and Shanghai petrochemical aromatics plant of similar equipment reboiler is also adopted in the tube side walk and high pressure steam, liquid shell side go, but not put into production for many years had leaked, the high notes, high pressure steam pressure, velocity, planning shortcomings that induced oscillation is the leakage of the primary elements of heat exchanger.
1.3 in the process of the production of defects

Welding quality is the key of the heat exchanger production.
Heat exchanger tube to tubesheet welding [4], the heat affected zone in on both sides of weld formation, it is thin position of the welding joint, simple residual deformation and residual stress, namely simple constitute the basic conditions of stress corrosion.
If meet the influence of the corrosive environment, such as in the environment such as H2S, OH – (austenitic stainless steel in Cl -, OH – such as environment), will the onset of stress corrosion cracking, constitute a joint leakage of heat exchange tube.
Gaps between tube and tube plate liquid there is inactive, and slot liquid concentration cell, crevice corrosion, will constitute a heat exchange tube joint leakage.
Tube to tubesheet welding structure features a put tight small circular welding seam, tube plate thicker, if it is no good welding technology, it is easy to form slag inclusion of weld root, bad fusion, cracks, porosity of welding defects.
In the process of running these defects is affected by the alternating stress can expand, make the leakage of channel development, causing leakage.
This has become a heat exchanger failure elements throughout.

Choose strength expanded joint of heat exchanger in expanding tube plate and the heat exchange tube should be proper hardness difference, otherwise the pipe springback is greater than the tube plate and easily form is not tight.
Else, when the space between the outer diameter of tube and tube plate tube hole larger, intention to reach lhe swelling, pipe must have a large deformation.
Tube in tube expanding deformation added together, the hardness has been added, so that the heat exchange tube hardness more than the tube plate, the expanding increasingly tight].
1.4 operating elements

Produce heat exchanger in the process of operation, because the characteristics of the technology itself may be due to technical operation is not standard in medium pressure instability, temperature shock impact thermal stress.
The operating pressure and temperature suddenly shake will cause pressure force of tube flange gasket seal face attack change, repeat cycle, the flange bolt looseness, seal failure.
Thermal shock is a great way of speed and impact on, a specific heat exhaustion larger temperature gradient, ductility can make the information lost and attack brittle fracture [6].
2 attacks leaked azimuth and factor analysis of the heat exchanger

To sum up, the onset of heat exchanger leakage may be in the planning, production and application of the elements of a link of the three links, also may be two or three link together the results of the effect.
Through the heat exchanger for many, will now let the cat out of the onset of bearing elements and their attacks as shown in table 1.

About different azimuth because of the different elements of the leakage, should choose different methods to solve them.
Plan elements causing failure, usually demand from the construction planning and renovation;
Before the onset of the production process of welding defects should be revised to improve;
It is no good operation is more simple to improve.

3.1 planning should pay attention to the question

(1) medium is perpendicular to the regulation of lateral activity is the primary source of fluid induced vibration and impact are the key elements of cross flow velocity shell side inlet side the top piece of the gap between the baffle plate and tube plate and baffle plate distance.
Under the condition of fluid pressure drop allows, shortening the distance between baffle plate, increasing the heat exchange tube diameter, can progress control of the inherent frequency, reduce the cross flow velocity, to prevent the intention of control oscillation.
Substitute baffle rod baffle support, can thoroughly solve the fluid induced vibration question.

(2) in addition to thinking when select material, pressure, temperature, medium elements on places that have requirements of temperature difference, effect of equipment and materials should also pay attention to choose line swell coefficient, thermal conductivity, plasticity of good information.
Choose in construction planning temperature compensator, as far as possible to eliminate and reduce stress converged on site, make flattering section transition, adopted with good heat preservation measures, to reduce the temperature difference between inside and outside wall, thermal stress, prevent because of heat stress and make the container onset of plastic deformation and creep.

(3) the reasonable selection of baffle plate distance, baffle plate distance is too big, is used for induced oscillation, and baffle plate distance is too small, rigid is too big, can make the heat exchange tube deformation harmony ability is low, onset temperature stress.
So can through the programming calculation, on the premise of ensure the onset of oscillation choose larger distance.
3.2 production should pay attention to the question

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