Yarway AT-18/28, AT-38/48 and TempLow 4300 Desuperheater

Overview

The Yarway AT-18/28 heavy-duty A.T.-Temp desuperheater, offered by Emerson under the Fisher Yarway name together with the standard-duty AT-38/48 and the cast TempLow 4300, is an insertion-style, angle-body desuperheater with integrated spray water control. It reduces the temperature of superheated steam to a set point by injecting atomised water through a variable number of spray nozzles. Because the actuator-driven piston opens more or fewer nozzles rather than throttling the water ahead of them, the water pressure at each open nozzle stays constant, spray quality remains nearly uniform over the operating range, and no separate spray water control valve is needed. Typical duties are process steam cooling, boiler superheater and reheater attemperation and auxiliary steam let-down stations.

Seven Star LLC supplies the Yarway AT-18/28 family to power producers, refineries, petrochemical plants and industrial steam users in the Sultanate of Oman and the GCC. Our Muscat engineers collect the steam and water conditions, pipe size, flange rating and actuator preference, and work through the Emerson sizing data with you so that the nozzle set, body material and actuator are matched to the station before a quotation is issued. We also provide engineering support in Muscat for spare spray cylinders, packing and trim.

Operating Principle of the Yarway AT-18/28

Inside the Yarway AT-18/28, water enters through the water flange and fills the probe above a stem and disk that seats against a hardened seat. When the temperature controller calls for cooling, the actuator strokes the piston downward. The primary disk seat opens first and is followed by a no-flow dead band; only after that travel does a lower disk edge begin to uncover the water inlet orifices of the spray cylinder. Separating shutoff from flow control in this way protects the seat from low-flow erosion, which is why Emerson quotes Class V shutoff as standard and long tight-shutoff life even in high-turndown service. Water turndown greater than 50:1 is achievable, and control within 6 °C (10 °F) of saturation is possible.

Each Yarway AT-18/28 nozzle has two parts, an orifice plate and a nozzle body. Water reaches the chamber behind the orifice through individual feed holes in the cylinder wall, accelerates through the orifice, swirls in the nozzle chamber and leaves through the end cap as a fine, symmetrical hollow-cone spray. The TempLow 4300 achieves the same swirl with a one-piece nozzle fed radially, so its spray heads are not interchangeable with AT heads. The water quantity follows the classic heat balance GW = GST (H1 – H2) / (H2 – HW), and secondary atomisation is helped by steam velocity and density, which is why Emerson notes that nozzle selection matters most below about 10 bar.

Engineering Design of the Yarway AT-18/28

The Yarway AT-18 and AT-28 are forged-body designs for medium to high pressure steam, with an NPS 3 (AT-18) or NPS 4 (AT-28) steam flange from Class 600 to 2500, or DN 80 / DN 100 from PN 100 to PN 400 to EN 1092-1. Water flanges are NPS 1 to 2 on the AT-18 and NPS 1-1/2 to 3 on the AT-28. The piston and stem are nitrided for galling resistance, and the hardened, nitrided piston rings with their liquid-tight slot allow controllable flow coefficients as low as 0.005 Cv. Body materials include F22, F91 and F347H, giving shell temperatures up to 593 °C (1100 °F). Maximum continuous water flow is 25 m³/h for the AT-18 and 50 m³/h for the AT-28.

The AT-38 and AT-48 use a fabricated construction for low to medium pressure, Class 150 to 1500 or PN 10 to PN 250, and can also be built in stainless steel as a liquid-into-gas injector. The TempLow 4300 has a WC6 cast body that mounts through an NPS 3 flange in steam lines of NPS 6 to 16, uses 6 to 21 vortex nozzles and needs water at least 3.5 bar above steam pressure; it is intended for one-for-one replacement of existing TempLow installations. Across the Yarway AT-18/28 family, spray cylinders unscrew from the probe so capacity can be changed without replacing the stem, disk or seat, all nozzles are vacuum brazed in place, packing is single graphite with an optional live-loaded set on the AT models, and Fisher 657C or 657R diaphragm actuators are typical.

Key Features

Steam system engineers choose the Yarway AT-18/28 family for the following reasons:

• Variable-number nozzle control keeps water pressure constant at each open nozzle, giving fine atomisation across the full load range.
• An integral spray water control valve with Class V shutoff removes the need for a separate water control valve and its pressure drop.
• Water turndown greater than 50:1 suits units that cycle between minimum stable load and full output.
• A no-flow dead band between the shutoff seat and the flow-control edge protects the seat from erosion at low flow.
• Interchangeable spray cylinders with six or nine equal nozzles, or characterised combinations, allow capacity changes in the field.
• Forged F22, F91 and F347H bodies on the AT-18/28 handle shell temperatures up to 593 °C and ratings up to Class 2500.
• Hardened stainless steel nozzles, with Alloy 6 and N07718 options, handle water-to-steam pressure differences up to 100 bar.
• Installation in vertical or horizontal pipe needs minimal headroom and no atomising steam.

Industrial Applications

In Oman the largest users of variable-spray desuperheaters are combined-cycle and cogeneration plants, where attemperators on HRSG superheater and reheater sections hold steam temperature to the turbine during load changes, and where bypass and auxiliary steam systems need let-down stations that cool steam for headers, deaerators and fuel gas heating. The Yarway AT-18/28 forged design suits these high-pressure duties, while the AT-38/48 covers process steam headers at refineries in Sohar and Duqm and at fertiliser, methanol and aromatics units. For a Yarway AT-18/28 station, Emerson recommends about 12 to 15 m from the injection point to the temperature sensor, 4 to 6 m of straight pipe to the first bend in systems up to 25 bar, a water strainer of about 100 μm, and a water-to-steam ratio within 10 percent below 15 bar.

Desalination plants with thermal sections, district cooling energy centres using steam-driven absorption chillers, and food and paper mills in the Rusayl and Sohar industrial estates also run desuperheating stations where the Yarway AT-18/28 family improves on fixed-nozzle sprayers. Seven Star LLC supplies the desuperheater with the steam equipment around it, including the Masoneilan 84000 SteamForm steam conditioning valve, the Yarway 751/761 repairable drip and tracer steam trap and the Spirax Sarco BSA3HP bellows sealed stop valve.

Technical Specifications

The following data for the Yarway AT-18/28, AT-38/48 and TempLow 4300 is taken from Emerson product bulletin D104714X012 (June 2025):

How to Order

To size a Yarway AT-18/28, AT-38/48 or TempLow 4300 desuperheater, send Seven Star LLC the steam flow, inlet pressure and temperature, required outlet temperature, spray water pressure and temperature, pipe size and flange rating, along with nameplate details of any unit you are replacing. For each Yarway AT-18/28 enquiry, our team in Muscat will work through the Emerson product bulletin with you, confirm nozzle configuration, body material and actuator, and return a formal quotation with lead time. The Emerson product bulletin can be downloaded from this page, and engineers across Oman and the GCC can contact our instrumentation and valve division for technical support at any stage.