Cable Pressurization Buffering

Transcription

1 Cable Pressurization Buffering What This Practice Is About Introduction This practice describes the various buffering practices that are used to protect the integrity of the pressurized cable network during scheduled splice and sheath openings. In most cases, the buffering procedures presented here will provide adequate protection both to the individual cable and to the entire system. Determinations on which method to use should be based upon the degree of protection required for the specific cable environment. It should be noted, however, that certain extreme situations may require modifications of the procedures presented in this practice. If situations that have not been described in the following paragraphs arise, contact the supervisor for assistance. As each buffering method is presented, suggestions are made regarding the relative effectiveness and merits of the procedure. Wherever applicable, both the pros and cons of a particular buffering application are discussed to assess the effectiveness of the method. Definition By definition, buffering is a method of protecting both the cable and the system during splicing activity by providing a supplemental source (or sources) of air. When a pressurized cable sheath is opened during splicing activity, a 0 Pounds per Square Inch (PSI) leak is created. Pressurized air escapes from the opening and cable protection is reduced. Prescribed buffering procedures maximize cable protection by reducing the size of the pneumatic section in which the splice is located (Figure 1). This is accomplished by adding a source of air to an existing pneumatic section. If a splice opening occurs close to a source of air (or on a cable with very low pneumatic resistance), the increased demand on the air system could cause a dryer failure. Proper placement of a supplemental air source can prevent this from happening. Specifically, the purpose of buffering a sheath opening is fourfold: Purpose of Buffering To provide immediate protection to the cable being spliced by maintaining pressure in those portions that are vulnerable when the sheath is opened. System Studies Incorporated (28102.DES) 1

2 Figure 1 Buffering to Reduce Pneumatic Section 2

3 To maintain pressure protection throughout the entire system by keeping delivery pressures at or above the minimum air pressure standards. To offset the increased air pressure demands of central office compressors during splicing activities by providing a supplemental source of pressurized air. To minimize alarms to the Cable Pressure Monitoring System (CPMS). Methods of buffering vary depending upon the type of pressurization system in use and specific buffering objectives. It is critical to understand that there are certain tradeoffs with each buffering application. For example, it is advisable to weigh the preparation time with what is being accomplished in the way of overall cable protection. Although this practice does not address these tradeoffs specifically, it does suggest which buffering methods provide the most cable protection and which provide the least. Buffering Applications Buffering is a method of minimizing pneumatic sections and providing dual feed protection to a sheath opening. If pressurized air converges on a sheath opening from opposite directions, the sections of cable on both sides of the opening will be protected. In each of the buffering applications described in this part, cable pressures are maintained and the system's principal air source is protected against increased demand. There are one or more buffering procedures for each of the following applications. (a) Buffering in single feed systems (b) Buffering in air pipe systems (c) Buffering with buffer pipe Single Feed Systems In a single feed system, the technician must reduce the pneumatic section in which the splice is located and establish dual feed protection to the cable. Depending upon the location of the original air source, adequate buffering is performed with either one supplemental air source or two. Buffering Near the Central Office If the splice location is 1,000 feet or less from the office meter panel, buffering can be performed with one nitrogen tank. In this application, the tank should be placed on the field side of the sheath opening opposite the original source (Figure 2). The tank should be placed close enough to the opening to reduce the pneumatic section as much as possible, but not so close that the tank will empty before splicing has been completed. The recommended tank placement distance is 1,000 feet from the splice opening. Placing a tank closer than 1,000 feet decreases the pneumatic resistance between the tank and the splice location and causes a significant increase in the tank's flow rate. System Studies Incorporated (28102.DES) 3

4 Figure 2 Buffering Near Meter Panel Note: The buffering technique in Figure 2 provides adequate protection to the cable in most situations. If the access hole were wet, however, it would also be advisable to pump the closest access holes on each side of the splice opening. Buffering Farther Out in the Field If a splice location is farther than 1,000 feet from the office meter panel, two nitrogen tanks are required to effectively reduce the pneumatic section and provide adequate protection to the cable. In this situation, one nitrogen tank is placed on the office side of the splice; the other is placed on the field side. Each air source should be placed approximately 1,000 feet from the splice location (Figure 3). Figure 3 Buffering in the Field Note: It should be understood that the 1,000 foot tank placement distance referred to throughout this practice is only an approximation. In practice, distances may vary by as much as 200 to 300 feet with acceptable results. Buffering Near a Remote Dryer The buffering applications described for locations near the central office also apply to a single feed system with a remote dryer as the principal air source. In a situation where a remote dryer is within 1,000 feet of the proposed sheath opening, it is recommended that the remote dryer be turned off and two nitrogen tanks be used to buffer the splice. In this situation, place two nitrogen tanks on the cable one on each side of the splice location. Each tank should be placed approximately 1,000 feet from the sheath 4

5 opening (Figure 4). Once the tanks have been positioned and turned on, shut off the remote dryer by closing the bypass valve on the 3/8" tubing at the back of the dryer. If there is a manifold at the dryer, turn off the air only to the cable being spliced. In the example in Figure 4, opening the sheath without shutting off the remote dryer could cause excessive loss of air and overload the dryer. Proper buffering, as described above, eliminates this danger and provides maximum protection to the cable. It is critical to reopen the bypass valve at the dryer (or turn on the air supply at the manifold) after splicing has been completed. Figure 4 Buffering Close to Source Air Pipe Systems There are several buffering procedures that may be used in an air pipe pressurization system. The distinction is generally based on the location of the existing air sources and where the buffering will take place. The explanations which follow address buffering in air pipe systems at the following locations: On single feed laterals On dual feed cable sections between air pipe manifolds On dual feed sections in the manifold access hole Single Feed Laterals In a single feed section, such as a lateral or toward the endpoint of the cable past the manifold location, a supplemental source of air is required on the field side of the proposed sheath opening to establish dual feed cable protection. A tank (or tanks) regulated to a maximum of 10 PSI should be placed approximately 1,000 feet from the splice opening. Figure 5 illustrates a situation where the last air pipe manifold on the pipe is located approximately 1,000 feet from the splice location. In this situation, only one supplemental air source is required on the field side of the splice (1,000 feet away) to buffer the cable. The air pipe manifold provides the air supply to the cable on the central office side of the splice. System Studies Incorporated (28102.DES) 5

6 Figure 5 Buffering on Single Feed Section 1,000 Feet from Manifold A variation of the example in Figure 5 is when the last air pipe manifold on the pipe is located within 1,000 feet of the splice location. In this situation, a nitrogen tank is placed on the CO side of the manifold approximately 1,000 feet from the splice opening; a second tank is placed on the field side of the splice (Figure 6). Figure 6 Buffering on Single Feed Section Less Than 1,000 Feet from Manifold In this type of buffering application, a tank should always be placed at the end of the cable even if the distance from the endpoint to the splice is less than 1,000 feet. Establishing dual feed protection during buffering is more important than minimizing supplemental air source usage. Once the tanks have been placed, the manifold air source to the cable is turned off and the sheath opened. Once splicing has been completed, make sure to turn the air pipe manifold back on again. Note: Under no circumstances should an air pipe manifold be used for buffering when the distance from the manifold to the splice is less than 1,000 feet. This is especially true when splicing takes place in the manifold access hole. In these situations the increase in air flow at the manifold could severely jeopardize delivery pressure to the other cables in the pipe system. If the proposed splice location is farther than 1,000 feet from the last manifold on the pipe, a third buffering application exists. In this situation, two supplemental nitrogen 6

7 tanks must be placed on the cable on opposite sides of the splice opening (Figure 7). Once again, each tank should be placed approximately 1,000 feet from the splice. Figure 7 Buffering on Single Feed Section Farther Than 1,000 Feet from Manifold Buffering Midway Between Manifolds Dual feed cable sections provide a number of buffering options. For example, there are three approaches to buffering at access hole locations midway between air pipe manifolds. Four different buffering methods may be used when a sheath opening is scheduled at a manifold access hole. In each situation, the objectives are the same: to reduce the pneumatic sections of the cables being spliced and to assure delivery pressure to the other pneumatic sections. The decision on which method to use must be based on the amount of protection needed for each specific cable environment. The various methods of buffering at locations midway between manifolds are explained below. The least protective method is described first; the most protective method is described last. The procedures in each application are essentially the same as those described in the Mechanics of Buffering section of this practice. Variations in the procedures pertain to the actual placement of the supplemental air source(s). Midpoint buffering practices are as follows: (1) When the proposed splice location is in a dry access hole between air pipe manifolds, buffer the air pipe with an auxiliary air source (Figure 8). This sometimes supplements delivery pressure to the other cables in the system. For the most part, however, it serves only to reduce pipe alarms to the Figure 8 Buffering Air Pipe Between Two Manifold Access Holes System Studies Incorporated (28102.DES) 7

8 CPMS. It does not increase protection to the cables being opened because it does not reduce the length of the pneumatic section. This method is the least recommended method of buffering when access holes are wet. Note: The delivery pressure of the nitrogen tank must be regulated to 0.5 PSI above the air pipe pressure. If the nitrogen tank is substantially higher, it could drain during splicing activity. (2) Place nitrogen tanks on the cable 1,000 feet away from the sheath opening to assure adequate cable protection (Figure 9). This is a better approach to buffering when wet access hole conditions exist. The advantage of this application over the first midpoint buffering method is that placing nitrogen tanks decreases the size of the pneumatic section. A potential problem is that one or both tanks may run dry. Figure 9 Buffering Cable Midway Between Manifold Access Holes (Least Protection) (3) Place buffer hoses from the air pipe to the cable 1,000 feet away from the sheath opening in both directions, and place a nitrogen tank in the access hole with the splice (Figure 10). The buffer hoses provide protection to the cables, and the nitrogen tank on the air pipe prevents CPMS alarms and supplements pipe delivery pressure. Figure 10 Buffer Hoses from Air Pipe to Cable 8

9 Note: The pressure at the tank should be set at 0.5 PSI above the pipe pressure. The advantage of this type of buffering is that, unlike nitrogen tanks, buffer hoses provide an unlimited source of air from the air pipe. The disadvantage is that buffer hoses are not always removed after buffering. Each buffer hose left in place creates at least one unmonitored pneumatic section. Buffering in the Manifold Access Hole When splicing activity occurs in the manifold access hole or within 1,000 feet of it, both the cable and the manifold must be protected. In this situation, a new dual feed pneumatic section is created, and the manifold air flow to the cable is turned off. Follow the procedures for buffering as explained at the end of this practice. Also, be sure to turn off the manifold valve for the cable being opened after the new air source(s) has been attached. Do not turn off the entire manifold. If the cable being spliced is stubbed or interlaced with other cables, their respective manifold valves must also be turned off during splicing activity. This should be done before opening the closure. Make sure that the manifold is turned back on when splicing activity has been completed and that all supplemental air sources have been removed. There are four methods for buffering a splice opening that is located at or near an air pipe manifold: (a) Use buffer hoses to supply air feed from the air pipe to the cable section (Figure 11). Place each buffer hose 1,000 feet away from the sheath opening. (b) Use nitrogen tanks to buffer the sheath opening direction. Place the tanks at the same locations (1,000 feet from the sheath opening) as the buffer hoses in Figure 11. Figure 11 Buffering Near Manifold with Buffer Hoses (c) If the splice opening is near the last manifold on the air pipe, place a nitrogen tank on the cable 1,000 feet to the field side of the sheath opening (Figure 12). Use a buffer hose from the air pipe to the cable on the central office side of the opening. System Studies Incorporated (28102.DES) 9

10 (d) Use a nitrogen tank in place of the buffer hose in Figure 12. In this situation, there will be two nitrogen tanks supplying air to the cable section each 1,000 feet away in the opposite directions. Figure 12 Buffering Near the Last Air Pipe Manifold Systems with Buffer Pipe Another acceptable method of buffering is the use of a specially installed air pipe called a buffer pipe. In dual feed systems, the buffer pipe is engineered into the pressurization system along with the main feeder pipe. A buffer pipe is a standard air pipe that parallels the main route. In an air pipe system it is placed in the same conduit run as the feeder pipe. Buffering Equipment Buffer pipe originates at a pipe panel in the central office and generally includes the following equipment: Flow transducer (0 20 SCFH) Buffer pipe identification tags Pressure testing valves in each access hole A control flow valve at the end of the pipe. A buffer pipe provides easy access to an air source during buffering activity. Depending upon the location of the sheath opening, the buffer pipe can eliminate the use of nitrogen tanks during buffering activities. As a ready air source, it is pneumatically connected to a cable with buffer hoses placed at the recommended distances (Figure 13). Note: As in other buffering applications at or near an air pipe manifold, it is necessary to turn off air feed from the manifold to the cable being spliced. Make sure to turn the manifold air supply back on after splicing has been completed. The Proper Use of Buffer Pipe The same general buffering procedures presented throughout this part apply to buffer pipe. Special emphasis, however, must be placed on not misusing the pipe. Buffer pipe is not an engineering alternative to leak locating; it should not be used to keep 10

11 Figure 13 Buffer Pipe Application cable pressures up. It is a temporary and convenient air source for buffering. Among the concerns when using buffer pipe are the following: Caution should be taken not to open the sheath if cable pressure falls below the minimum acceptable pressure standards. Buffer hoses should not be placed closer than 1,000 feet from the splice opening. Doing so increases the output of the central office compressor without increasing cable protection. When splicing activities have been completed and the sheath closure has been tested, buffer hoses must be removed. Buffer pipe is not designed to be used as a permanent air source. If buffer hoses are not disconnected from the cable, a new unmonitored pneumatic section is created (Figure 14). Mechanics of Buffering The Manual Process of Buffering The manual process of buffering a sheath opening is essentially the same for each type of buffering application. This process includes placing a supplemental air source (or sources) on the cable or air pipe, checking pressures, opening the splice closure, maintaining air protection to the cable on each side of the opening, closing the sheath, and removing the supplemental air source(s). Variations of these procedures depend on each buffering application they are used in. For example, when splicing activity takes place near an existing air dryer, it may be necessary to turn off the air dryer to protect it from excessive output. In this case, an alternate source of air is required to replace the output of the dryer. System Studies Incorporated (28102.DES) 11

12 Figure 14 Unmonitored Pneumatic Section Buffering Procedure For the most part, however, buffering procedures are similar for each application. The recommended procedures are as follows: Step 1 Before opening the cable, measure the cable pressure with a C pressure gauge and record the reading. If the cable pressure does not meet or exceed minimum air pressure standards for the cable environment, contact the local supervisor for specific buffering procedures. Note: Minimum cable pressure standards are as listed below. Select the appropriate standard for your operating company. Underground Cable 5 or 6 PSI Buried Cable 3 or 4 PSI Step 2 Step 3 Step 4 Place a nitrogen tank or tanks on the cable at the prescribed locations. Connect a hose from the tank to an F pressure testing valve at each buffering location. Regulate delivery pressure from the nitrogen tank(s) to 10 PSI. Pressures higher than 10 PSI may result in damage to the C pressure gauge and cable. Open the splice closure. 12

13 Step 5 Step 6 Step 7 Step 8 Step 9 Periodically check the pressure level of the nitrogen tank(s), and replace with a new tank(s) if necessary. When work has been completed, use a torque wrench for proper sealing of all closures. Pressure test the sheath closure according to local practices. Continue buffering until the cable pressure at the sheath opening is within 0.5 PSI of the original pressure. Remove nitrogen tank(s) and/or buffer hose after the cable pressure at the opening has been restored. Note: Although safety regulations are not discussed in this practice, it is critical that routine and specialized safety precautions are taken when working with high pressure equipment and air sources. Always wear safety glasses and double check all fittings before and after making connections to the cable system. Fittings must be able to withstand the normal operating system pressure and the slightly higher buffering pressure. When using nitrogen tanks, be careful not to bleed excessive nitrogen into the access holes. Nitrogen can displace the available oxygen and seriously contaminate the work environment. System Studies Incorporated (28102.DES) 13

14 14