Steam System Operation
STEAM SYSTEM OPERATION
Case Study: In 1970s one refinery designed for energy efficiency in the 1970s uses high pressure steam to power turbines that exhaust to a medium pressure header for process heating. The cold eyes perspective of an energy specialist showed refinery management how changes in run plans and operating conditions had resulted in steam system imbalances. Some medium pressure steam was being produced from a high pressure header let down station. Further, the low pressure header was balanced by a partially open let down station from the medium pressure header. Most alarming, low pressure steam was venting to the atmosphere. Not only was the refinery essentially venting high pressure steam, it was also losing highly valued polished water. Once identified, the problem was easily solved within hours and at no cost. This refiner also misstated the marginal economics among his three steam systems. Although projects and operating plans were based on precise four-digit costs for each utility system, use of accounting data masked true economics. With a let down station open between high and medium pressure headers, what is the difference in marginal value for a pound of steam from each header? The answer is zero, and favoring the use of medium pressure steam has no incremental value over high pressure steam as long as the valve remains open.
Common Causes of Flange Leakage
Uneven Bolt Stress.
Improper Flange Alignment
Improper Gasket Centering
Dirty or Damaged Flange Faces
Excessive Piping System Loads at Flange Locations
Improper Gasket Size or Material
Improper Flange Facing
COMMON CAUSES OF PIPING VIBRATION
ACOUSTICALLY INDUCED VIBRATION
High capacity, gas flow, and pressure reducing systems are more vulnerable to acoustically induced vibrations. The pressure reduction can occur at control valves, at restriction orifices, safety valves, or when sonic flow occurs at a branch connection to a header. Failures can occur in only a few hours since the higher structural and acoustical natural frequencies are excited, and the material endurance limit can be reached in a short time. Failures have occurred in steam desuperheater systems, compressor recycle letdown systems, and safety letdown systems. Severe vibration has also occurred in pipeline pressure letdown systems.
1. Screening a system to determine if it is prone to acoustically induced vibration.
2. Calculating sound power levels throughout the piping system downstream of the pressure reducer.
3.Comparing the calculated sound power level to a design limit that is typically based on pipe diameter and thickness.
4. Treating the system if the sound power levels are excessive. Treatment alternatives include reducing the sound power level at the source, improving the structural integrity of the system, or some combination of methods.
SURGE INDUCED VIBRATION
The transient pressure variation occurs in the piping by the sudden alteration of steady state velocity is called hydraulic surge or water hammer. The pressure surge moves through the pipe at the speed of sound.
Potential consequences are
Excessive internal pressure
Large pipe movements.
Common causes of surge
Classic water hammer.
Vapor pocket collapse
Safety valve blowdown into a line full of liquid
Pump startup and shutdown
Slug flow leads to flow-induced vibration in two-phase fluid systems.
In a horizontal line, the vapor above the liquid can travel much faster than the liquid. This creates waves at the liquid surface and entrains some of the liquid into the vapor stream. At high vapor rates, slugs of liquid form across the pipe cross-section and travel at speeds that approach the vapor velocity. When this occurs, a wide range of reaction forces can occur at pipe bends, depending on the size of the slugs that are formed. Reaction forces developed at pipe bends due to slug flow can cause excessive piping vibration and movement unless the piping system is adequately restrained.
WIND INDUCED VIBRATION
Piping vibration caused by wind can lead to vortex shedding from the pipe surface. If wind strikes at a right angle to the axis of a cylinder, aerodynamic forces due to vortex shedding occur at the following frequency that is a function of wind velocity, cylinder diameter, and Strouhal number (0.18 for cylinders in air).These forces act on the pipe at right angles to the wind direction. Although the forces are small, the amplitude of vibration may be large if the shedding frequency is close to the natural frequency of the piping.In control such situation, the stiffness and the natural frequency of the piping should be increased by adding bracing, consistent with still meeting piping flexibility and associated equipment requirements. Mechanical snubbers and shock absorbers may also be used to change the stiffness and add damping to the piping system while still permitting its thermal movement.
Earthquakes leads to piping vibration either directly due to resonance or by the motion of pipe supports or equipment connections. Piping in earthquake prone areas should be checked for forces due to earthquakes.
Universal Engineering Services has more than a decades experience in the pressure vessel design for clients in UAE, Oman, Qatar, Kuwait, Bahrain, Saudi Arabia, and many more countries all over the world. UES work to many ASME standards to design and validate pressure vessels, boiler, fittings and piping systems. We are also experts in Stress analysis of piping, structural design, ASME Joint review, Design of Storage tanks, code calculations, FEA/FEM, and spotless service on design management.
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