Special Considerations for Tubesheet in High-Temperature HRSGs

Introduction:

In High Temperature / High Flux Heat Recovery Steam Generators (HRSG), especially those where gas temperatures surpass 500°C and can soar up to 1100°C, specialized considerations come to the forefront. A prime example is the design of fire tube-type HRSGs, tailored to accommodate hot gases in channels through the use of special materials or refractory lining for extreme temperature applications. This article delves into the unique challenges and strategies associated with tubesheet design, focusing on instances like the steam reformer effluent Waste Heat Boiler (WHB) found in Ammonia, Methanol, and Hydrogen Plants.

Thin Tubesheet in High-Temperature HRSGs:

A defining feature of high-temperature firetube HRSGs is the thin tubesheet construction, typically ranging from 15 to 40 mm. However, the use of standard tubesheet configurations, such as those outlined in TEMA or ASME Part UHX, is cautioned against in high-temperature, high-flux (severe service) firetube HRSGs. The concern lies in the potential for excessive metal temperature gradients in the tubesheet, leading to high stresses.

To address this, the thin tubesheet design of WHBs relies on stays to provide necessary reinforcement (for stiffened type) or having knuckle toa allow for expansion and bending of tubesheet (in flexible type), particularly in flat portions without tubes. Efficient cooling of the tubesheet hinges on effective heat transfer at the tubesheet back face, achieved through shell-side vaporization of water and high local circulation rates.

In general circulation ratio for firetube horizontal HRSG is much higher than water tubes boiler by factor or 2 to 3 at same operating pressure.

Flexible and Stiffened Thin Tubesheet

Thin tubesheet can be either flexible or stiffened. Flexible tubesheet incorporates knuckle at the junction with shell allowing for bending under the differential thermal expansion between tubes and shell, because of that flexible tubesheet can be used for relative short tubes (normally less than 6 m). It can be imagined that knuckle acts as an expansion bellow.

In case boiler running dry, differential thermal expansion will be relatively high causing high bending stress at the Knuckle area and repetition can lead to fatigue failure. Also, plugging tubes (based on thinning or in service leak), means that those specific plugged tubes will not expand similar to other tubes as hot gases will not pass inside it and consequently it will be as anchor points within the flexible tubehseet leading to high localized stresses at the plugged tube and adjacent tubes. ?

Thin stiffened tubesheet is reinforced by stiffener plates which allows the tubesheet to be always flat without bending (not flexible), that is why there is no limitations on tube length and no restriction on differential thermal expansion between tubes and shell. In many case tubes can be as along as 12 m.

That is a tailored design requires numerical analysis using Finite Element Analysis for the design of the tubesheet. Stiffener palates are normally huge and significantly increases boiler weight. It can also impact the water circulation at the backside of the hot tubesheet.

The hot face of the tubesheet is safeguarded by a refractory lining, further minimizing tubesheet temperature by restricting heat flow with the use of insulated ferrules inserted in each tube inlet. These ferrules, crafted from high-temperature resistant metallic or ceramic materials, wrapped with insulating material, ensure a snug fit in the tube bore.

Closing the crevice between the tube and tubesheet is of paramount importance, and this is accomplished through an in-bore weld configuration.

Thick Tubesheets in Vertical WHBs:

For designs involving thick tubesheet, commonly used in vertical WHBs in synthesis loop in Ammonia plants, U-tubes with hot and cold ends are strategically arranged. This arrangement, with the hot shank surrounded by cold shanks, offers advantages such as maintaining tubesheet and hot end tube wall temperatures inside the tubesheet below 380°C. This helps avoid nitriding and allows for the use of ferritic tubes.

Consequently, the inlet ends of the tubes inside the tubesheet, as well as the entire tubesheet itself, align with gas outlet temperatures.

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References and bibliography:

1. API RP 534; Heat Recovery Steam Generators
2. API RP 669; Heat Recovery Systems
3. BORSIG; Process Gas Waste Heat Recovery Systems
4. Thyssenkrupp; Ammonia Technology
5. FbmHudosn