2000 Hook-up Book

Draining Steam Mains

Draining Steam Mains Note from the example that in most cases, other than large dis tribution mains, 1/2" Thermo Dynamic ® traps have ample capacity. For shorter lengths between drip points, and for small diameter pipes, the 1/2" low capacity TD trap more than meets even start up loads, but on larger mains it may be worth fitting par allel 1/2" traps as in Fig. II-6 (page 86). Low pressure mains are best drained using float and thermo static traps, and these traps can also be used at higher pressures. The design of drip stations are fairly simple. The most com mon rules to follow for sizing the drip pockets are: 1. The diameter of the drip pock ets shall be the same size as the distribution line up to 6 inches in diameter. The diam eter shall be half the size of the distribution line over 6 inches but never less than 6 inches.

2. The length of the drip pocket shall be 1-1/2 times the diam eter of the distribution line but not less than 18 inches. Drip Leg Spacing The spacing between the drainage points is often greater than is desirable. On a long hori zontal run (or rather one with a fall in the direction of the flow of about 1/2" in 10 feet or 1/250) drain points should be provided at intervals of 100 to 200 feet. Longer lengths should be split up by additional drain points. Any natural collecting points in the systems, such as at the foot of any riser, should also be drained. A very long run laid with a fall in this way may become so low that at intervals it must be elevat ed with a riser. The foot of each of these “relay points” also requires a collecting pocket and steam trap.

Sometimes the ground con tours are such that the steam main can only be run uphill. This will mean the drain points should be at closer intervals, say 50 ft. apart, and the size of the main increased. The lower steam velocity then allows the conden sate to drain in the opposite direction to the steam flow. Air venting of steam mains is of paramount importance and is far too often overlooked. Steam entering the pipes tends to push the air already there in front of it as would a piston. Automatic air vents, fitted on top of tees at the terminal points of the main and the larger branches, will allow dis charge of this air. Absence of air vents means that the air will pass through the steam traps (where it may well slow down the dis charge of condensate) or through the steam using equipment itself.

SYSTEM DESIGN

Figure 5 Draining and Relaying Steam Main

Fall 1/2" in 10 Ft

Steam

Steam Trap

Steam Trap

Steam Trap

Steam Trap

Steam Trap

Condensate

Case in Action: Steam Main and Steam Tracing System Drainage

The majority of steam traps in refineries are installed on steam main and steam tracing systems. Thorough drainage of steam mains/branch lines is essential for effec tive heat transfer around the refinery and for waterhammer prevention. This holds true for condensate drainage from steam tracing lines/jackets, though some degree of back up (or sub-cooling) is permissible in some applications. The predominant steam trap installed is a non repairable type that incorporates a permanent pipeline connector. Scattered throughout the system are a number of iron and steel body repairable types. Most notable failure of steam traps are precipitate for mation on bucket weep-holes and discharge orifices that eventually plugs the trap shut. A common culprit is valve sealing compound injected into leaking valves which forms small pellets that settle in low points, such as drip legs/steam traps and on strainer screens making blow down difficult. This problem also occurs during occasional “system upset” when hydrocarbon contaminants are mis takenly introduced to the steam system. A noise detector and/or a temperature-indicating device is required to detect trap failure. Especially costly is

the fact that operators are not allowed to remove traps for repair when threading from the line is required. Maintenance personnel must be involved. Solution Universal connector steam traps were installed for trial in one of the dirtiest drip stations at the refinery. The traps held up under adverse operating conditions requiring only periodic cleaning. Since the time of installation, all failed inverted bucket traps in this service were replaced with universal connector traps. Strainers were installed upstream of each. Benefits • The addition of Thermo-Dynamic ® traps allowed for eas ier field trap testing. • The addition of universal connectors significantly reduced steam trap installation and repair time. • 33% reduction in steam trap inventory due to standard trap for all sizes. • Reduced energy loss is significantly reduced using Thermo Dynamic ® steam traps versus original inverted bucket traps.

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