Steam trap failures

In some applications such as stripping steam for separation columns, steam cannot be recovered as condensate returns to the deaerator. In other cases, condensate is lost due to either steam trap failure or lack of a recovery facility. Extra amount of freshwater must be provided as makeup to compensate for the loss. There are two major debits in costs for the makeup water one is water treatment cost and the other is preheating cost to heat the freshwater from local temperature to condensate temperature. In the previous steam cost calculations, the costs for producing treated makeup water is evenly distributed on every pound of steam generated from boilers and cascading down to MP and LP steam. 

Most of the failure modes that affect steam traps can be attributed to variations in operating parameters or improper maintenance. Table 22.1 lists the more common causes of steam trap failures. 

High reliability. Many things can go wrong with a steam tracing system but, very few of the potential problems lead to a heat tracing failure. Steam traps fail, but they usually fail in the open position. 

Partial waterlogging of heater batteries can lead to early failure due to differential thermal expansion. Steam trap selection should take account of this. 

Figure 8.2 shows a common type of reboiler failure. The steam trap on the condensate drain line has stuck open. A steam trap is a device intended to open when its float is lifted by water. The steam trap remains open until all the water drains out of the trap. Then, when there is no more water to keep the trap open, it shuts. But, if the float sticks open, steam can blow through the steam trap. This is called a blown condensate seal. The average vapor velocity through the tubes... 

We can see, then, that either condensate backup, or blowing the condensate seal, will cause a steam reboiler to lose capacity. If you think either of these two problems could cause a loss in reboiler duty, try opening the bypass around the steam trap. If the reboiler duty goes up, the problem was condensate backup. If the reboiler duty goes down, then the problem might be a blown condensate seal. If it looks like a blown condensate seal problem, close the steam trap bypass. Then, partially close the valve downstream of the steam trap. If this increases the reboiler duty, a blown condensate seal failure is proved. 

The downstream piping must be adequately sized to effectively handle this volume. An undersized condensate returnline results in a high flash-steam velocity, which may cause waterhammer (due to wave formation), hydrodynamic noise, premature erosion, and high backpressure. The latter condition reduces the available working differential pressure and, hence, the condensate removal capability of the steam trap. In fact, with some traps, excessive backpressure causes partial or full failure. 

Failure in steam traps not only cause steam losses but also lead to damages to equipment and safety via water hammer. Two types of water hammers could be caused by steam trap malfunetioning the cavitation occurs when a trap valve sticks in the open position as the hot steam bubbles collapse in the cold condensate, while the slug type happens when a trap valve sticks in the closed position as slugs of condensate remain in steam lines. Some traps tend to fail in an open position, while others tend to fail in a closed position. This tendency may also be influenced by the characteristics of the steam system. 

It is important to know that steam traps should not be sized based on pipe size. A common problem is poorly sized traps that cause premature failure through excessive cycles and wear on internal parts as well as excessive steam leaks. Therefore, steam traps are selected primarily on duty second, on duty variation for transient cases and third, on equipment requirement (Kenny, 1989). Proper installation and maintenance together with regular vendor service can maintain good steam trap performance and long life. 

Purchaser may provide a bypass line on the discharge side to check the flow by measuring it separately before feeding in to the process unit Provision of Internal safety and rupture dise (on the discharge side) along with instantaneous trip for drive motor if excessive disehatge head is encormteied due to closed valves and ehoked lines (e.g. solidified sulphur in pipes due to failure of steam traps)... 

Table 22.1 Common failure modes of steam traps...

A common failure mode of steam traps is failure of the sealing device (i.e., plunger, disk, or valve) to return to a leak-tight seat when in its normal operating mode. Leakage during normal operation may lead to abnormal... 

Component failure Steam traps, undergroimd steam lines, electrical hnes and substations, elec trical and electronic components and modules, insulation foam, fiberglass and refractory, seals, low and high temperature, doors, ports, windows, cooling towers, heat exchangers, plmnbing hnes and systems, motors, pumps, ventilators, bearings... 

The propellant was simply pent-up water and steam pressure. The destructive pressure buildup occurred as the compressor operated for over five hours with no cooling water flow. Cooling water was trapped in the water jacket on the compression head as the compressor piston continued operating. The team concluded that the fragmentation of the compressor water jacket was a consequence of the operating condition, and a metal failure analysis of the compressor head was not necessary. The damage was immediate and limited, there was no release of gas, and the incident did not require the attention of the available on-site emergency squad. Fortunately, there were no personnel injuries due to this incident. 

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