Process Steam Traps

The best steam trap will not function correctly if the piping to and from it is not properly designed and installed. Many factors are important and some will be discussed. 

Multiple trapping of equipment is often provided in the interest of space and investment economy. Generally, each of the heat exchanges connected to the common trap operate at sufficiently different conditions to result in poor drainage of condensate from one or more of the parallel equipment. A single trap should never be used to drain more than one piece of equipment. Even though the equipment may be identical In design, they will not operate identically and the steam flows will not divide equally. 

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Undersized condensate return-lines create one of the most common problems encountered with process steam traps. Hot condensate passing through a trap orifice loses pressure, which lowers the enthalpy of the condensate. This enthalpy change causes some of the condensate to flash into steam. The volume of the resulting two-phase mixture is usually many times that of the upstream condensate. 

During this process some water will have condensed in the steam-trap D and also in the distillation bulb F. If at the end of the steaming-out process, the Bunsen burner is removed from the generator A, the pressure in A will be reduced owing to steam condensation, and the liquid in F will be sucked back into D provided that the benL-over tube is carefully adjusted, the bulb F may be almost completely emptied of liquid as desired. Finally the condensed water in the steam-trap D may be run out by op ing the tap Tj. 

Condensa.te Return Systems. Ia a process plant, steam traps are used to drain and return condensate. Given proper appHcation and continuous maintenance, these can operate with minimal steam leakage. Correct iastaHation is also important (12). 

For draining principal items of process equipment, level-controUed condensate chambers provide much better performance and rehabiUty than steam traps. Usage is generally justified when condensate flow is greater than 4500 kg /h. 

PH — Process Heater or Furnace R — Reactor S — Separator St — Strainer ST — Steam Tutbine Str — Steam trap SV — Safety Valve Tr — Trap V — Valve... 

From an examination of the process flowsheet the man-hours total 685 for the significant equipment. Items such as steam traps and miscellaneous small time-items can be omitted from the total. Includes 75 man-hours for pipeline sizing. 

Steam traps are installed in condensate, mechanical return systems and are a frequently overlooked item for reducing operating costs. Large industrial process plants typically have many hundreds of steam traps installed to recover low-energy condensate and remove (potentially corrosive) air and carbon dioxide. 

It is easy to improperly design a steam trap. The design must work for two circumstances and often a designer will check only one of these. The circumstance often overlooked is as follows On startup or upset, the steam control valve can open wide so that the steam chest pressure (assume for this discussion that we are speaking of a reboiler) rises to full steam line pressure. At a time like this, the steam trap downstream pressure can be atmospheric due to process variations or the operators opening the trap discharge to atmosphere in an attempt to get it working. 

Services for water, air, clean and house steam, water return and drain S. Steam Traps, multiple stainless-steel 316L traps guarantee the sterilization temperature Is maintained In all process lines... 

Figure 3.9. Steam heaters, (a) Flow of steam is controlled off the PF outlet temperature, and condensate is removed with a steam trap or under liquid level control. Subject to difficulties when condensation pressure is below atmospheric, (b) Temperature control on the condensate removal has the effect of varying the amount of flooding of the heat transfer surface and hence the rate of condensation. Because the flow of condensate through the valve is relatively slow, this mode of control is sluggish compared with (a). However, the liquid valve is cheaper than the vapor one. (c) Bypass of process fluid around the exchanger. The condensing pressure is maintained above atmospheric so that the trap can discharge freely, (d) Cascade control. The steam pressure responds quickly to upsets in steam supply conditions. The more sluggish PF temperature is used to adjust the pressure so as to maintain the proper rate of heat transfer.

For steam applications, it is generally neither recommended nor usual to use manifolds or headers, but they can be utilised if proper consideration is given to all aspects of the design and installation, and proper draining is provided via, for instance, automatic steam traps. On the other hand, considering current environmental or safety requirements, a lot of process vapours are manifolded into flare headers before being flared off or otherwise treated and disposed of. 

Some of the renewable energy processes operate under high pressures (Fy, and others such as thermal solar systems circulate oils under high temperatures. In these and many other applications, leakage in tanks (under- or aboveground), valves, steam traps, or pipes must be detected. Table 3.107 lists the detectable leak rates of some common gases, including H2. 

Production of heat energy at Rohm Haas has been improved by better control of combustion in boilers, cleaning and maintenance of boilers and process heaters to maintain their efficiency, elimination of steam leakages, steam trap maintenance, and improved condensate recovery (4). 

Determine the condensate load. The first step in selecting a steam trap for any type of equipment is determination of the condensate load. Use the following general procedure. a. Solid materials in autoclaves, retorts, and sterilizers. How much condensate is formed when 2000 lb of solid material with a specific heat of 1.0 is processed in 15 min at 240°F by 25-psig steam from an initial temperature of 60°F in an insulated steel retort ... 

A wet steam at 20 bar with a quality of 0.97 (see Problem 7.32) leaks through a defective steam trap and expands to a pressure of 1 atm. The process can be considered to take place in two stages a rapid adiabatic expansion to 1 atm accompanied by complete evaporation of the liquid droplets in the wet steam, followed by cooling at I atm to ambient temperature. may be neglected in both stages. 

Steam-heated calandrias with process boiling temperature less than 100°C can present control problems, especially at reduced rates and during start-up. In most such cases, low-pressure steam is used for heating. Control is usually achieved by throttling the entering steam in order to reduce the pressure at which it is condensed. At reduced rates this often results in steam pressures less than atmospheric or less than the steam condensate return system pressure. The steam is usually removed through steam traps which require a positive pressure differential to fiinction. In order for the trap to function, steam condensate floods part of the steam chamber imtil the steam pressure is sufficient to operate the trap. This leads to poor control and all the problems associated with condensate flooding. 

VALVES. A typical processing plant contains thousands of valves of many different sizes and shapes. Despite the variety in their design, however, all valves have a common primary purpose to slow down or stop the flow of a fluid. Some valves work best in on-or-off service, fully open or fully closed. Others are designed to throttle, to reduce the pressure and flow rate of a fluid. Still others permit flow in one direction only or only under certain conditions of temperature and pressure. A steam trap, which is a special form of valve, allows water and inert gas to pass through while holding back the steam. Finally, through accessory devices, valves can be made to control the temperature, pressure, liquid level, or other properties of a fluid at points remote from the valve itself. 

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