Design Methods for Group Trapping for Steam

YungPlantEng · Jun 18, 2024

Most manufacturers recommend against group trapping due to backpressuring of the separate legs and potential flooding / hammer.

Are there cases where group trapping is either possible or preferred? We have a trace system where there are a large amount of vertical nozzles requiring heat trace. These all relieve via thermodynamic disc traps to a common condensate return to boiler feed water tank, which poses reliability issues when they fail open causing flooding.

It has been difficult to monitor trap reliability due to location in the plant and the vast amount of traps there are on the tank. A senior technician (who I regard as probably one of the most intelligent person I have had the privilege of working with) asked whether there was a way to simplify the amount of traps required through group trapping from a common header.

TLV raises some points about potentially using an oversized header to reduce issues but I was wondering if there was any concrete literature or studies on this.

TBP · Jun 18, 2024

Spirax Sarco has (had?) some excellent literature on steam systems, one of which is "Steam Heated Tracer Lines", which was a booklet (11 titles in the set - but I only have 3, courtesy of a Scottish salesman what worked in the UK before coming to Canada) published in the UK. The section on "Steam Trapping" leads-off with "This in general should always follow the rule. One tracer - one trap."

If you don't have a copy of "Hook-Ups - Design of Fluid Systems" by Spirax Sarco, see if you can find one. The sales reps used to give them away to customers. I've used it since the mid 1980's and it's never let me down.

mk3223 · Jun 18, 2024

It won't work to have several steam lines to use one common steam trap for recycling the condensate. Normally, the steam trap piping should be used for at each condensate collection points, which connected to a condensate collection header. The condensate header size should be adequate for a two-phase flow condition.
The steam/condensate system is an important water & energy conservation operation for the plant. A testing valve may be included at the stream trap outlet to facilitate a test of the trap.

FacEngrPE · Jun 18, 2024

Group trapping will result in some of the trap lines getting air bound, which decreases the effectiveness of the tracing.

Orifice plate steam traps might be an option. Somewhat higher steam consumption, longer service period. Some risk of clogging the small orifice.
Trace lines are typically on or off, so sizing does not need to consider part load case.
More information here - Orifice Plate Steam Traps Link

shvet · Jun 19, 2024

YungPlantEng said:
Are there cases where group trapping is either possible or preferred?

Group trapping is related to the dawn of industrialization when only one design of steam trap was available called "steam pot". Unreliable, large, heavy and expensive one comparing to cost of steam consuming equipment. At that time negative consequences of group trapping were tolerable.
Process undistry is very conservative so outdated practices are still sometimes considered as an option available.

Drain header said:
If equipment is constructed in such a way that group trapping is already installed and it is not possible to trap individually, then sometimes an equalizing drain header will be installed before the trap. This drain header must have a large enough vapor space capacity to enable pressure balancing which will help condensate drainage.
Installing a drain header may help some condensate flow, but most likely causes higher radiation steam loss, larger and more expensive pipes and valves, and must be a near-perfect installation to avoid coil stratification. Drain headers cannot resolve problems related to uneven pressure within piping, and this is why individual trapping remains the preferred method.
This is not recommended if individual trapping can be done, and there are several instances where results can be less than desirable. For example, if the drain pipes feeding the header are not always gravity draining downward (there should be no lifts) and if they are not large enough to allow balancing within them, then steam from another section which has higher pressure than the section with a higher load will cause back-up.

A drain header only has a chance to work if the inlet piping supplying it is large enough to allow equalization by condensate draining down the piping sides with steam balancing through the center of the outlet. If there is the chance that the inlet piping supplying the drain header closes off the line with condensate, then condensate will back-up leading to coil stratification, uneven heating, and other waterlogging issues may develop.
While installing a sufficiently large header can help resolve some drainage problems related to group trapping of condensate equipment – provided the above conditions are met – the most effective way to optimize performance of a heat exchanger is to individually trap sections that may have unique pressures from adjoining sections, even if the steam supply is common.

https://www.tlv.com/steam-info/steam-theory/problems/group-trapping

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Design Methods for Group Trapping for Steam

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