State of the art In chemical and petrochemical industry rather frequently the process fluids are corrosive substances to carbon steel or low-alloy steel ; it is therefore necessary to utilize, i for t ; e construction of the equipment where such corrosive ! substances are handled, other materials than carbon steel or low-alloy steel.

It is known in the art that materials which are corrosion , resistant, such as titanium, zirconium, tantalium and the like are very expensive and also present a low mechanical resistance : therefore their use in the equipment manufacture is highly unsuitable and uneconomical particularly in high pressure conditions. Furthermore some of these materials may present difficulties in the welding i stage. i It is therefore common practice to manufacture the f equipment handling corrosive fluids with a pressure resistant body. made in carbon steel or in low-alloy steel Il with an internal corrosion resistant lining with the purpose of protecting the whole pressure resistant body from entering into contact with the corrosive fluids.

The most common type of equipment is represented by

reactors, heat exchangers, vessels, columns and the like where the pressure resistant body is internally lined with a corrosion resistant material.

Different methods are known by which such internal lining is performed : by weld overlay or by explosion cladding of the lining onto the pressure resistant body or by a lining I formel by a number of sheets shaped each to adhere as much I as possible to the pressure resistant body and welded il together to each other (loose lining).

The pressure resistant body can be of any known type in the art: solid wall, multi-layer, multi-wall, etc.

Should any leakage happen, through the sheets or, most likely through the weldings between the sheets of the lining, the corrosive fluid corrodes the carbon steel or the low-alloy steel not only in the area close to the leakage, but it may corrode also a much larger area due to the fact that it may also spread in the interstitial gap i formed'between the lining and the pressure resistant body being the contact between them not a sealing contact. I The corrosion : nay compromize the stability of the equipment making it unusable or, even worth, causing the explosion of the equipment with economical damages and/or even injuries or loss of human lives.

In order to make evident when a leakage occurs in an equipment lined with a number of sheets adherent, but not sealed, to the pressure resistant body (loose-lining), a number of weep-holes are provided in the pressure resistant <BR> <BR> <BR> <BR> ody mainly in correspondence to the sheet circumpherential<BR> I and longitudinal welding areas. The weep-holes go through

the whole thickness of the pressure resistant body from the external side to the internal lining and they also are i prote ted from corrosion by small pipes made of the same anticorrosive ! material as the lining. The purpose of these weep-holes is to remove any possible corrosion fluid which may get into contact with the pressure resistant body and, mainly, they represent the possibility for the plant operators to check by visual inspection the possible existence of any leakage and consequently to take the necessEary steps to prevent damages.

Details concerning the positioning of the weep-holes, generally located close to the circumferential and I longiludinal weldings of the lining sheets, are in accordance with plant technology and/or with the technology and practice of the equipment manufacturer. Consequently they ill be not discussed, herewith.

Detecting a possible leakage of corrosive fluids from the weep-holes, which are generally in a large number and positioned in plant sites difficult to be reached, represents a severe duty for the plant operators and its ; reliability lies also upon their diligence.

This method for detecting leakages of fluids from a weep- holes, has the disadvantage that, should the leakage be very scanty, the operator's observation may fail to detect them. In plants which handle fluids that may undergo crystallization during expansion, leakages of the corrosive ! I fluids may, when expanded, cause plugging of the weep- holes, so nullifying the visual inspection of the

operators.

A typical case is represented by the urea plant wherein corrosive fluids, kept inside the equipment under pressure at 140-250 bar and at a temperature of 190-210°C, when leaking out, may crystallize so plugging the weep-holes.

It is known that in order to make this check easier, in some plants, all the weep-holes are provided with small glass or transparent plastic bottle like vessels which contain a reagent able to change colour in the presence of a specific undesired compound. II i The drawback of this detecting method is represented by the fact that the operators should reach every weep-hole ; furthermore frequently a change of colour may be detected in these small vessels due to factors different from the presence of process corrosive fluids.

In order to make easier the task of the operators 'controlling the various weep-holes of the equipment, several small diameter pipes were connected to the weep- I holes and made them drawing inside small bottles, placed at round-level, the detection being relied on the change of colour'of the reagent in the presence of a specific ndesi oed compound. he dawback Of this detecting method, based on colour changing, is the same as above, i. e. the change of colour may be due to factors different from the process fluids.

Furthermore the number of the checks to be carried out represents not an easy task to be accomplished by and, most likely, a crystallization of the fluid may also occur inside the thin, long pipes causing a possible consequent

plugging.

In the attempt to improve the method avoiding formation of plugs, inside the pipes, nitrogen gas was introduced therein and then bubbled inside the detecting reagent solution. Nitro en bubbling inside the detecting solution causes a i I certain loss of the reagent solution which needs to be frequently refilled.

In urea plants another method of detecting possible leakages of corrosive fluids consists in insufflating air through a weep-hole connected with one or more different weep-holes in order to provoke coming out of the process fluid, if any, which will be then detected by known I techniques. further method for detecting leakages from high pressure equipment having an internal loose lining is described in German'Patent 19740340 C2. The detecting device, installed an in each weep-hole, essentially consists in a small tube, made of the same material as the lining, wherein the !' ossible leakage of process corrosive fluid, mainly I consisting in a liquid or a gas, provokes the closure of an i electric circuit and consequently a bulb lightens and/or an alarm is operated. When the corrosive fluid is a gas, it a moves a piston, positioned inside the tube, by which the electric circuit is also closed. This last method of detection presents some drawbacks which make it not reliable when : 1) the amount of gas leakage is very small and so it may happen that the piston will be not ooved (nough to close the electric circuit ; 2) the amount of gasij leakage getting cooler from the inside to the

outsiAe of thl equipment may be transformed into liquid or solid'form and both forms cannot close the electric circuit; 3) the piston may be blocked due to a friction against the walls of the housing tube; 4) any interruption, in the electric circuit, due, for example, to electric wire corrosion, may give no signals even in the presence of leakages. It is an object of the present invention to provide an automatic highly sensitive and reliable system for detecting and monitoring leakages from high pressure equipment, internally lined with a loose lining, such as reactors, heat exchangers, columns, vessels and the like containing fluids corrosive in respect to the pressure resistant body, whose integrity is vital to the plant n I operation as well as to the safety and life of operators. Such detecting and monitoring system is characterized by a 'continuous operation and is completely free from the drawbacks shown by the mentioned known methods. Furthermore the system is practically free from operating and maintenance costs and does not require any active supervision from the operators. In particular, the detecting and monitoring system of the i i nvention is essentially formed by a number of detectors hich re inserted into each of the weep-holes of one or ore equipment present in a plant handling corrosive fluids. Each detector is electrically connected to a relay, located close to the equipment under control, by which possible visual and/or acoustic signals are made evident to the operators. Said detectors may be connected even in

groups in parallel, for example 6 to 10, to a single relay, so that the number of relays is the same as the number of detectors or lower.

All the relays are grouped together in a central connecting box: each detector or a group of them, is identified by a number by which, in case of an operated alarm, the involved detector or the group of detectors is disclosed. Another object of the present invention is represented by the connection between the central connecting box and the control board located. in the plant control room, continuously watched during the plant operation, where electric/electronic signals are transferred or incorporated in a DCS system. A further object of the invention is represented by an autotesting electric circuit able to provide information about the integrity of the electric circuit itself. A part of this electric circuit, having a particular resistance, and the most external part of the detector are very close to each other and contained in a small box which may be of Explosion proof type. till a further object of'the invention is represented by he possibility to verify from the control board, located in the control room, on a specific or on a scheduled request, the functioning of the detector and the integrity of the circuits. The detecting and monitoring system of the present invention can be installed in new as well as in already existing equipment. Detailed description of the invention I

The object of the invention is described into details with reference to the Figures 1-7 which are given to provide a better comprehension of the invention without limiting it. Figure 1 shows the block diagram by which the different partslq of the detecting and monitoring system of the invention are each other connected.

Figure 2 represents in more details the detecting and monitoring system generically indicated in Figure 1: in particular it is clearly evidenced the connection between the weep-hole and the detecting part of the system (detector), partially inside a detector box which can be of the explosion proof type, as well as the connection between the detector and the other parts of the system. t Figure 3 is an axonometric view of the detector box of Figure'2. Figure 1 4 reprlesents a side view of the detector box of Figure 2. Figure 5 represents the circuit of Figure 2 in three different situations : a normal functioning of the equipment and two upset conditions. Figure 6 represents two portions of the electric wire inside the detector : one inside the weep-hole and the other i inside the detector box, both being protected, in part, by a perforated sheath. Figure 7 represents an element of the detector consisting n a small plastic piston. i In a, high pressure equipment (1), having a pressure ! resistant body (2) and an internal loose lining (3) resistant to corrosion, the ! detection box (4) of the system

is connected to the equipment (1) through the weep-hole (5) and, through an electric connection (6), to a central connecting box (7), able to send out a visual and/or an acoustic alarm signal, which in turn is connected, through an electric/electronic circuit (8), to the control board (9) in the control room of the plant where the alarm signals are repeated on the control board or in a D. C. S. system. I The central connecting box (7) is connected to a plurality of circuits (6) each corresponding to a single weep-hole !, (5) or to a group of weep-holes and contains a plurality of relays (10), one for each weep-hole or for a group of weep- , ! holes. A small tube (11), at one of its ends, is connected with the weep-hole (5) by means of a connecting piece (28) and it contains, at the opposite end, surmounted by a bracket , (16), a suitably shaped hard plastic piston (12), which can slide inside the tube (11) to a limited extent, but is prevented from entirely enter it. In a preferred embodiment of the invention, the piston (12) is made from teflon. long the full length of the small tube (11), inside of it, ! an electric wire (13) is located, which is entirely tnsulaßed exceyt for the portion in proximity of the weep- hole and inside it as well'as in proximity and inside the piston (12) ; said non-insulated portions of the electric wire (13) are partially covered with the perforated sheaths (17) and (18). One end of the wire (13) crosses also the piston (12) and, through the screw (14), holding the solenoid (24), together with the spring (25) and the

electric resistence (20), it forms part of the electric circuit (6).

While said screw (14) is electrically connected with the positive pole of the circuit (6), the tube (11) is connected to the negative pole of the same circuit. When ai leakage occurs a by-pass (15) becomes closed in the circuit (6) and a higher current intensity circulates Ii therein and operates a visual and/or acoustic alarm in the relay (10) and a visual and/or acoustic alarm in the control room of the plant. In the presence of a leakage essentially gaseous, the pressure moves the piston (12) and the screw (14) gets in contact with the bracket (16) : the same by-pass (15) is , closed and the above visual and/or acoustic alarms are operated. The particular characteristics of the electric wire (13), with the perforated sheaths (17), (18), allow the detection f a small quantity (droplets) of liquid and even of a small quantity of gas : in this case the gas cools down I moving'away from the equipment and it may form droplets of liquid and even solid humid crystals that also may close the by-pass (15) in the circuit (6). The piston (12) carries an 0-ring (19) which reduces the friction between the piston (12) itself and the internal wall of the tube (11), when moving due to the presence of gas from leakage. Inside the detector box (4) is located a resistance (20) as a part, of the circuit (6) that, in normal conditions, maintains said. circuit (6) closed, with an operated visual

signal in the relay (10), for example, a bulb (21)). This I corresponds to the situation of the left circuit of Figure i 5. The same Figure 5 clearly indicates the circuit in the presence of a leakage (central circuit) by lightening of a bulb (22) and even (right circuit) the breaking of the , electric wire by lightening of bulb (23) (autotest of the circuit (6) ). A similar test concerning the circuit (6) can be effected by intentionally closing the by-pass (15) from the control board (9). The bulbs (21), (22) and (23) have convenient different colours. These signals can all be transferred to the control board (9) by means of the electric/electronic circuit (8) and transformed in visual and/or acoustic signals. 11 the materials that may come into contact with the i leakage fluidshave to be resistant to corrosion in respect g, i p to leakage fluids. Also the electric wire (13) of the circuit (6) is made from corrosion resistant material. The detector box (4), preferably, may be made from hard plastic material. In order to test the piston (12) being free to slide inside the tube (11), the solenoid (24) can be operated from the control board (9) and the piston (12) moved against the bracket (16) so provoking a temporary closure of the by- ass of the circuit (6). Should the piston (12) be moved only of a small stroke, the I spring' (25), mounted on the bracket (16), helps it close i the circuit and may also, in particular positions, push it back towards its resting position. In alternative, the spring (25) can be mounted on the screw (14) as indicated

in Figure 7.

In case of a strong leakage the hole (26), suitably located on the small pipe (11), provides the discharge of the fluid into the detector box (4) and therefrom to the atmosphere through the safety plug (27) positioned in the bottom part of said box, to prevent overpressure therein. The detector box (4) may be installed with the tube (11) in vertical position as well as in horizontal position. This possibility may be convenient, should the detecting and bai monitoring system of the invention be installed in an 0 existing equipment and the room around the equipment be rather small. Example The detecting and monitoring system object of the present invention was tested in equipment of an existing urea plant having the capacity of 1, 500 T/D. A urea plant is composed by several sections characterized by different operating conditions : pressure, temperature and composition of fluids. the most serious conditions are those existing in the high pressure section where high pressure (140-250 bar) and high temperature (190-210°C) are present together with highly corrosive fluid streams. Said corrosiveness is mainly due to the presence, in the process fluids, of ammonium carbamate, an intermediate compound in the urea production process from ammonia and carbon dioxide. Due to these reasons the high pressure equipment of urea plants are generally lined, most frequently with a loose lining. In said plants the only four equipment (reactor, stripper,

carbamate condenser and carbamate separator) are provided with a total number of weep-holes of about 120. Each weep- hole is connected with an explosion proof detector box (4) and all of them were connected (not individually, but joined togethqr six by six) to the relay central connecting i i ;,, box (7) from'which an electric connection to the control board (9) provides suitable alarms in case of leakages. All the materials that, due to a leakage, may get into contact with the corrosive fluid of the plant, are AISI 316 urea grade. The electric wire (13) of the circuit (6) is made from the same corrosion resistant material to avoid corrosion also from the sorrounding ambience. The alarms given by the system during the operating period of the : plant proved to really correspond to an effective leakage in the plant. The leakage was so revealed even if i n extremely Ismall quantity, for example, when due to mlicrocraks, hard to be detected by known methods. Contrary to other known methods, the detecting and monitoring system of the invention resulted to be very sensitive and reliable, practically free from operating and maintenance costs and without any comsumption of reagents. Furthermore the duty of the operators to check all the weep-holes was practically reduced to zero.