Transportation and erection of Pressure Vessels
- Posted by: César Boró Martín
- Category: Static Equipment
First of all, it should be considered that transport and erection operations of pressure vessels could be focused from two different points of view, which are intrinsically related. On the one hand, the pressure vessel must not be damaged under no circumstances, since the moment the equipment leaves the manufacturer’s workshop until it is installed in its location and position. To accomplish this, several means are used. Nevertheless, on the other hand, those means have to comply with various requirements, including legal ones.
The range of possible scenarios during the transportation and erection of pressure vessels is relatively wide, mainly because it depends on various factors, such as the distance from the origin up to the delivery point, the routes to be used to transport the pressure vessel, the necessary transport and erection means, the equipment dimensions and materials, among others.
The equipment, during transportation and erection activities, could have to withstand high mechanical stresses that would involve permanent strains, implying significant impacts in economic and planning parts. Therefore, all of those stresses have to be considered, both when the pressure vessel is being designed against the mentioned stresses and for the design of the necessary auxiliary components.
It should be noted that long and thin wall pressure vessels, such as a slender column, and equipment with singular geometric zones, such as a vessel with a conical transition, are more susceptible to be unwantedly damaged than, let’s say, a reactor whose wall is much thicker.
Each mean of transportation (ship, truck, etc.) presents a different stress structure that will affect the equipment. For instance, a pressure vessel transported by sea will be impacted by oscillating loads because of the waves.
Logically, and as required by the codes usually used for the design of pressure vessels, all the loads that could cause damages in the equipment have to be considered in the calculations. At this point, it is necessary to emphasize that pressure vessels operating in vertical position require to be verified regarding transport activities that, in the case of a horizontal vessel, could be covered by the calculations under operation conditions, depending on the case.
One of the most commonly used ways to transport any pressure vessel, including horizontal ones, is to support it on saddles, which must withstand the loads that equipment will transmit, but especially, saddles must not impose stresses higher than allowable ones in the vessel shell . The quantity and location of transport saddles will depend on the vessel configuration and the means of transport used. Saddles could be manufactured of wood or steel, as long as corrosion is not an issue in the case of steel saddles. The transport saddles and the equipment must be attached as a single piece. As a general criterion, it can be said that it is convenient to locate the saddles close to stiff elements of the pressure vessel, such as, for instance, dished heads or stiffening rings.
A vertical equipment supported on transport saddles could, in some cases, be calculated as a horizontal vessel supported on service saddles. Therefore, the calculation procedure will have many similarities, such as, avoiding that bending stresses or those that occur in the horn of the saddles exceed the allowable values.
With regard to the pressure vessel and its elements, the calculation must consider every component used for fastening during transportation or erection, such as, , lifting lugs, trunnions, tailing lugs, etc. Also, the loads that these elements transmit to the zone of the equipment where they are attached (shell, heads, skirts, legs, etc.) must be analyzed. For all these cases, it is necessary to guarantee that induced stresses do not exceed the allowable ones.
The movements to which the pressure vessel is subjected during the transport activities involve loads in the three directions. Vertical loads can occur in the same or in the opposite direction as weight, but horizontal loads will occur in the longitudinal and/or transverse direction.
Auxiliary elements are those that are used to tie the pressure vessel, very important devices. Lifting components of the equipment are normally used to lash it and they must be considered in such a way that they absorb longitudinal and transverse loads.
When performing a pressure vessel erection analysis, the designer must consider several variables. As a general rule, a vertical equipment will be transported horizontally, so it will be necessary to consider that the vessel that is erected will be in all the angles from the horizontal to the vertical position, and stresses derived from every angle will affect the vessel. It is also necessary to point out that this operation has a potential safety hazard for people, so it is usual that lifting calculations are done considering a safety factor that minimizes the possibility of an accident.
It is a normal practice that pressure vessels are transported with the center of gravity clearly marked on the equipment, so that the loads distribution is adequate during these activities.
Regarding the pressure vessel packing and protection against corrosion, equipment are usually filled by means of an inert gas (normally nitrogen) to avoid any issues related to this subject.
Up to this point, the focus has been put in adapting the transport and erection methods to avoid damages and to guarantee the integrity of the equipment, complying with applicable laws and technical limits in the route, during the transport, or in installation site, during erection, etc. However, sometimes, the people involved in these subjects face the problem, not as infrequent as one would expect, that the pressure vessel configuration makes it impossible to transport the equipment or to erect it without breaching the established limits. In these cases, consequently, the vessel design has to be adapted to achieve that the vessel is transported and erected without problems.