In a tube and shell heat exchanger, such as a vertical tube chemical reactor, there are various situations in which it is desirable to temporarily plug or indicate the condition of one or more tubes. A temporary plug may be used to protect a tube or catalyst inside the tube, or to identify the condition of the tube, or to aid in keeping track of the work progress on tubes while the reactor is out of service for maintenance. In the prior art, this is usually done by a colored plastic cap or plug inside the top end of each tube, with a particular cap or plug color intended to identify a particular tube condition (such as a tube to be unloaded, cleaned, loaded, pressure-drop tested, tube failed due to high pressure drop, tube failed due to a low pressure drop, tube passed the pressure drop test, tubes that have or will have thermocouples or pressure sensors, or even tubes from which catalyst samples may be removed for laboratory analysis after the catalyst has been used).
This procedure requires large numbers of caps or plugs of different colors to be used as the tubes in the reactor are worked on to indicate the status of particular tubes and to serve as a visual indication of the work flow progress on all of the tubes in the reactor while the tubes are brought to within the desired specifications for cleaning, empty, full, outage, and pressure drop. It is not unusual to run out of different colors to designate the numerous tube conditions, resulting in improvisations by the catalyst changeover crew in order to identify the condition of the tubes. For instance, a cap or plug may be removed from a tube and if pliable enough then twisted and reinserted sideways into the top edge of the tube to designate the new condition of the tube. Sometimes two caps or plugs are stacked on top of each other and inserted into a tube. In both of these cases, the caps or plugs then project awkwardly from the tube and are more prone to being accidentally kicked and dislodged from the tube, so they no longer perform the desired function of visually identifying the tube condition. In the case of the twisted and reinserted cap or plug, it can become lodged down inside the tube if walked upon, such that it may not be easily seen and may even unknowingly be left behind.
In order to prevent the caps or plugs from being accidentally dislodged from, or pushed down into the tubes, they usually are designed to fit snugly inside the tubes. Some caps or plugs are simple pipe thread protectors that are applied to protect the threads on the ends of threaded pipes that use a tapered thread like those available from Caplugs from Buffalo, N.Y. (www.caplugs.com). Since these devices are tapered, they can be pushed very securely into a tube. Upper tubesheets with caps or plugs in them are continuously walked upon by the catalyst change crew, and heavy machines and supplies are temporarily located and moved across this upper tube sheet, forcing the caps or plugs securely into the tubes.
Cap or plug size availability is often somewhat limited, as these caps or plugs are sized for commercially sized pipe threads, although custom sizes could be used. Reactor tube inner diameters are seldom standard pipe sizes, and instead are often custom sized to optimize heat transfer. In the case of tight fitting tapered caps or plugs, this results in two problems. First, the tight wedging action of the tapered cap or plug within or upon the tube makes it very difficult to remove the cap or plug from the tube, so it usually is pried but with a screwdriver, which may damage the cap or plug and may even score the inside wall of the tube. Second, after a cap or plug which has been removed and reinserted a number of times may become; cut or otherwise damaged by the blade of a screw driver, or by being removed by narrow end (needle nose) pliers, it may no longer fit snugly inside the tube, making it more prone to being accidentally kicked out or otherwise, dislodged from its tube, or it may be damaged so that it is no longer serviceable.
Tape, such as duct tape, can be attached to the caps or plugs in certain tubes in order to identify them (this tape may be accidentally scuffed off or otherwise removed from its corresponding cap or plug). Sometimes caps or plugs are actually taped to the tubesheet (resulting in art undesirable sticky residue on the tubesheet when the tape is removed).
Most tube caps or plugs, such as those intended to protect the threads on the ends of pipes, include an open cavity or depression where dirt, catalyst and foreign material can accumulate. It can be difficult and time consuming to remove the dirt, spilled catalyst and foreign material from these cavities, especially when many thousands of such caps or plugs are used.
Some tube plugs have a flat top (do not include an open cavity) but they incorporate a tapered body that fits snugly into the tubes and require a screw driver or fork device to remove them, making them time consuming and sometimes difficult to install and remove. Other plugs have a flat top but have a loose fitting body so they effectively float around in the tube. These plugs are relatively easy to install and remove, but they do not seal the tube from air flow, and they are sometimes inadvertently removed while walking across them or by moving equipment across them. Since reactors are often made of carbon steel, the reactor is often kept warm to prevent condensation that could lead to iron oxide formation. Also, it is desirable to keep most catalyst pellets that have been loaded dry and away from moisture. The warm vertical tubes in the reactor vessel may induce natural convection air flow through the tubes, and this may be undesirable as the air being supplied to the bottom of the tube could be moisture-laden ambient air. Another reason for minimizing or eliminating this natural convection phenomenon is that some catalyst can off-gas certain chemicals which can be annoying and even hazardous for the catalyst handling crew. Also, convection air flow can raise the ambient temperature in the dome area above the tubesheet, making it uncomfortable for the catalyst handling personnel. Loose fitting plastic caps with or without recesses or plugs with flat tops can slow down the convection air flow, but they do not stop it.