Steam drum

New Inspection Method of Steam Drums in Paper Mills. 

In Austria, as well as all over Europe, the first and repetition tests of all pressure equipments including steam drums are required for security reasons within fixed time intervals. These repetitive inspections are done differently in the most European countries, but most time these inspections include, according to the European Pressure Equipment Directive" and the specific national law any kind of over-pressurisation (e.g. hydrotest) and visual inside inspection. 

One production line of a paper mill consists according the size and the quality of the produced paper sometimes from more than 50 steam drums to dry and flatten the produced paper. These drums (cylinders with flat bottoms, see figure 1) will be used with a steam pressure up to 500 kPa (5 bar) and additionally with a rotation speed up 1200 m.min the material is mainly grey cast iron (with lamellar graphite). The diameters can reach up to 2.2 m and the cylindrical lengths up to 10 m. For the specific flattening drums the cyhndrical diameters can be up to 5 m and more. 

A fatal accident and some other disasters, which were caused by small cracks, lead to a more strict consideration of the security of these steam drums. Parallel to these the economical pressure, due to the globalisation of the today s industry, lead to the increase of the pressure and the rotation speed of the paper production machines for a higher output of the production, which means, that all safety aspects from the design and the material will be exploited totally. On the other hand cast iron is also not a ductile and comfortable material, like the most steels for the pressure equipment. 

Based on our practical experiences in the applieation of AE on pressure equipments since 1979, we started in the year 1994 to apply AE on steam drums in combination with pneumatic tests. Before we are able to do this, we have to perform extensive lab tests with the specific material (grey cast iron and cast steel) from which these drums are produced. 

Beside all technical reasons the big advantage of a pneumatic test is, that the steam drums can remain within the line because first we have no additional load for the bearing and only small adjustments (for the connection with the pressurisation unit and the tightening of the man ways for the applied low temperature gas test) have to be done to make the drum ready for a pneumatic loading. The pressurised air is available in every paper mill and even if the maximum pressure does not fit, the use of a compressor or pressure bottles produce no problems. 

How it was declared, the new steam drums cannot undergo a hydrotest, because they loose their roundness and balance due to the additional weight of the water, even if the cylindrical part will be supported. 

The older steam drums, which can sustain these additional effects better, have to be removed of their position in the production line because of the bearing and this would be expensive and time consuming. 

Since 1994 when the test method was implemented in the inspection program of many paper mills, we test now regularly the steam drums in the most paper mills in Austria (see table 1). 

Table 1 Steam drums, which undergo a pneumatic test monitored with AE...

It was pointed out, that the periodical inspection of the steam drums has been become an absolute must especially under the circumstance, that the economical pressure results in smaller wall thickness, higher steam pressure and higher rotation speed. The conventional periodical inspection (hydrotest and visual inside inspection) is on one hand time consuming and therefore expensive and on the other hand the results of the hydrotest are doubtful and can result in a seriously damage of the roundness and balance of the steam drum. 

In a ouce-throiigh system, the feedwater entering the unit absorbs heat until it is completely couveided to steam. The total mass flow through the waterwall tubes equals the feedwater flow and, during uorm operation, the total steam flow As only steam leaves the boiler, there is no need for a steam drum. 

The failure took place in a large water-tube boiler used for generating steam in a chemical plant. The layout of the boiler is shown in Fig. 13.1. At the bottom of the boiler is a cylindrical pressure vessel - the mud drum - which contains water and sediments. At the top of the boiler is the steam drum, which contains water and steam. The two drums are connected by 200 tubes through which the water circulates. The tubes are heated from the outside by the flue gases from a coal-fired furnace. The water in the "hot" tubes moves upwards from the mud drum to the steam drum, and the water in the "cool" tubes moves downwards from the steam drum to the mud drum. A convection circuit is therefore set up where water circulates around the boiler and picks up heat in the process. The water tubes are 10 m long, have an outside diameter of 100 mm and are 5 mm thick in the wall. They are made from a steel of composition Fe-0.18% C, 0.45% Mn, 0.20% Si. The boiler operates with a working pressure of 50 bar and a water temperature of 264°C. 

Ellison has published an extremely important factor for steam drum design called the Drum-Level-Stability Factor. As manufacturers have learned how to increase boiler design ratings, the criteria for steam drum design have lagged. The three historical steam drum design criteria have been ... 

This fourth criterion can be met at a low steam drum cost. Only one percent of the cost of the boiler spent on the steam drum can provide it. The fourth criterion is met by requiring that the Drum-Level-Stability Factor (D.L.S.F.) be equal to 1.0 minimum. When this exists the steam drum level will be stable for wide and sudden operational changes. 

Vs, V,s = Specific volume of a pound of steam or water at saturation temperature and pressure of the steam drum at operating conditions. 

Steam drum pressure = 925psig C (from manufacturer) = 18.5 G (from manufacturer) = 6,000gal HR (from manufacturer) = 160,000 BTU/EPRS V, =1,500 gal... 

This steam drum level would be very stable (D.L.S.F. well above 1.0). 

Ellison, G. L., Steam Drum Level Stability Factor, Hydrocarbon Processing, May 1971. 

The partial containment system had many rooms in one room, with a pressure capability of about 26 psi, was the reactor core. The steam drums were in two rooms four main recirculation pumps were in each of two rooms. 

Figure 6.2-2 shows its operation. A mixture of 14% steam and 86% water from the pressure tubes went to steam drums, used to separate steam from liquid water with steam on top and liquid on the bottom. Steam drives the turbine, leaving at reduced temperature and pressure, condensing in the condenser and combining with liquid from the steam drier as feedwater for recycle to the reactor. 

The reactor had a loop for each half of the core. Each circuit had two steam drums and two. 500 MW(e) turbines. Water was circulated in each of the two circuits by four pumps in parallel (8 lotal), though usually each circuit had three in use and one on standby. The pumps supplied a t omplex of pipes under the reactor that fed water to the separate pressure tubes. 

At 0119 10, the operator began to increase the rate of feedwater return to reduce the recirculation flow to increase the water level in the steam drums. At 0119 45, the reduced inlet water. stopped water from boiling in the core. The absence of the steam voids reduced the reactivity, and control rods were withdrawn, such that only 6 to 8 rods were in the reactor, rather than the required 30. Then, to avoid reactor trip from steam drum or feedwater signals, their scram circuils v ere locked out (a safety regulation violation). 

A mechanic was affected by fumes while working on a steam drum. One of the steam lines from the drum was used for stripping a process column operating at a gauge pressure of 30 psi (2 bar). A valve on the line to the column was closed, but the line was not slip-plated. When the steam pressure was blown off, vapors from the column came back through the leaking valve into the steam lines (Figure 1-3). 

Section 1.1.4 described how fumes got into a steam drum because it was not properly isolated. Even when service lines are not directly connected to process materials, they should always be tested before maintenance, particularly if hot work is permitted on them, as the following incidents show ... 

A similar incident occurred in a steam drum in which steam was separated from hot condensate. On this occasion, the operator noticed that the pressure had risen above the set-point of the relief valve and tripped the plant [19]. 

Dampf-erzeuger, m. steam generator, boiler, erzeugung, /. steam generation production of vapor. -farbe /. Dyeing) steam color. -fSi bereit /. steam printing, -fass, n. steam drum. 

Construction is a water-cooled wall combustion chamber connected to a steam drum at high level. The bottoms of the walls are connected to headers. Sometimes a bottom or mud drum is incorporated, but improved water treatment now available does not always necessitate this. 

Superheater elements are connected to inlet and outlet headers. The inlet header receives dry saturated steam from the steam drum of a watertube boiler or the shell of a horizontal boiler. This steam passes through the elements where its temperature is raised and to the outlet header which is connected to the services. A thermometer or temperature recorder is fitted to the outlet header. 

In modem boilers, both FT steam spaces and WT steam drums incorporate various devices to promote the effective separation of steam from water and the production of dry steam. These devices include horizontal separators and baffle plates in the steam/water space, and chevron driers, cyclone separators, and secondary steam scrubbers in the steam space. Older or simpler boiler designs with steam release velocities of below 3 ft/s (0.9 m/s) may rely solely on the natural separation of steam from water. 

In WT boiler steam drums, the normal water level is at or slightly below the horizontal centerline to maximize the steam-to-water surface area. 

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