Method of pipeline interior drying

Abstract

Pipeline transport of hydrocarbons detect the moisture accumulation location along the pipeline and increase effectiveness of the drying process. Drying air moisture content reduction is obtained by the installing additional intermediate air drying devices in by-pass lines at block valve station along the pipeline to be dried. Purging is proceeded until normalized dew point temperature value of the outcoming air being achieved. Purging is interrupted for 12 hours or more, both upstream and downstream ends of pipeline leaving hermetically closed, and resumed with intermediate air drying devices disabled, continuously measuring the moisture content of the outcoming drying air, fixing the moment when the moisture accumulation in the surrounding air.

Claims

1. Method of drying a mainline pipeline by purging of the pipeline with drying air with subsequent humidity measurement at a downstream end of the pipeline, characterized in that moisture content in the drying air is reduced during purging by means of air drying devices, which are installed at bypass lines of linear valve stations of the pipeline to be dried; wherein purging is proceeded until normalized value of dew point temperature of the drying air at the downstream end of the pipeline to be dried between minus 15 C. and minus 30 C. is achieved, then purging is interrupted for at least 12 hours; next, purging of the pipeline to be dried is resumed with the air drying devices switched off, continuously measuring the moisture content in the drying air at the downstream end of the pipeline to be dried, wherein a timepoint which evidences of presence of moisture accumulation where the moisture content in the drying air exceeds the normalized value of the dew point temperature is registered; then a distance between water accumulation location and a pipeline upstream end of the pipeline to be dried is calculated, water is removed from an interior of the pipeline to be dried at water accumulation locations and purging is of the pipeline to be dried is continued until normalized value of the dew point temperature of the drying air at the downstream end of the pipeline to be dried is achieved.

Description

(1) Drying unit (1), comprising compressor and air-drying unit, e.g. MDU 7000 by Munters (Sweden), is connected to the upstream end of the mainline pipeline section being dried by flexible hoses (2), connected to flanges at the temporary blind plug (not shown) mounted at the upstream end of the mainline pipeline section being dried. At by-pass lines (3) contained in block valve stations (4) air drying devices (5) are installed and connected to gas extraction risers (6), existing on each block valve station (4). Meanwhile, line valve (7) and by-pass valves (8), (9) are closed, providing air passage through air drying devices (5) only. As air drying devices (5) cold regeneration adsorbers may be used, e.g. Dry Xtreme ND series (production of MTA Group, Italy), which are selected for drying of the specific section of the linear portion of the mainline pipeline, on the basis of throughput capacity of air drying devices (5), inlet and outlet diameters of air dryer and drying unit's capacity. At the downstream end of section of the linear portion of the mainline pipeline to be dried on-stream hygrometer (10) is installed which measures outgoing drying air dew point temperature by water. Next, drying unit (1) is switched on and drying air is passed through the section of the linear portion of the mainline pipeline to be dried. Wherein drying air moves at each of the linear valve stations (4) passes through the air drying device (5), which leads to a reduction of air moisture content, increasing its capability of water absorption at its way after valve station (4) in mainline pipeline interior, which provides reducing drying duration of the whole section to be dried. As the value of the dew point temperature corresponding to the normalized value (15 C. . . . 30 C.) is achieved at downstream end of section of the linear portion of the mainline pipeline to be dried, pipeline purging process is interrupted for period not less than 12 hours. During above mentioned 12 hours all line valves (7) of the linear valve stations (4) are opened and air drying devices (5) are switched off at the section of the linear portion of the mainline pipeline to be dried, providing drying air to pass along pipeline through line valves (7). After 12 hours (or more) hours purging of the section of the linear portion of the mainline pipeline to be dried is resumed along with simultaneous continuous measurement of the dew point temperature of the drying air at the downstream end of the section of the linear portion of the mainline pipeline to be dried. Continuous measurement of the dew point temperature of the drying is carried out during time (t.sub.control), required to displace the air contained in the interior of the section of the linear portion of the mainline pipeline to be dried, which is determined using the formula

(2) $\begin{array}{cc}{t}_{\mathrm{control}}=\frac{D^2{L}_{\mathrm{pipeline}}}{4q_{\mathrm{DrUn}}}& \left(1\right)\end{array}$

(3) where D is inner diameter of the section of the linear portion of the mainline pipeline to be dried, meters;

(4) L.sub.pipeline is the length of section of the linear portion of the mainline pipeline to be dried, meters;

(5) q.sub.DrUn is the capacity of the compressor of the drying unit (1), m.sup.3 per min.

(6) If during the time t.sub.control dew point temperature at the downstream end of the section of the linear portion of the mainline pipeline to be dried does not exceed the normalized value, purging is stopped and drying of the pipeline is considered to be completed. If the dew point temperature at the downstream end of section of the linear portion of the mainline pipeline to be dried exceeds the normalized value by the value higher than the measurement error of on-stream hygrometer (10), which indicates the presence of a moisture accumulation, than distance from the moisture accumulation to the upstream end of the section of the linear portion of the mainline pipeline to be dried is calculated using the formula

(7) $\begin{array}{cc}{X}_{\mathrm{moist}}=L_{\mathrm{pipeline}}-\frac{4t_{\mathrm{ex}}{q}_{\mathrm{DrUn}}}{D^2}& \left(2\right)\end{array}$

(8) where t.sub.ex (minutes) is the time elapsed since the purging resumption until registering of dew point temperature exceeding the normalized value. Next, the distance obtained is correlated (snapped) with the technological scheme and the profile of the route of the section of the linear portion of the mainline pipeline to be dried and the presumptive reason of the water accumulation occurrence is defined, e.g. low lay of land, connection between parallel pipeline routes or linear valve stations location. If technically feasible, the remaining water is removed from the pipeline interior, for example, by draining through the drainpipe or by pumping out with a pump. Next, the line valve at the linear valve station nearest to the water accumulation along the drying air flow is closed. Purging of the indicated section of the linear portion of the mainline pipeline to be dried is continued. Wherein releasing of the drying air is carried out by opening tap valve (11) through the vent stack (12) of the indicated linear valve station with bypass valve (8) open. If several water accumulations are detected on the section of the linear portion of the mainline pipeline to be dried, closing of line tap valves and the removal of the water are performed sequentially, starting with the accumulation closest to the upstream end of the section of the linear portion of the mainline pipeline to be dried. Next, all the line tap valves are opened and purging is continued until achieving the dew point temperature value at the downstream end of the section to be dried which is below or equal to the normalized value. The pipeline drying process is interrupted for period not less than 12 hours, after if necessary, all operations of the drying process are repeated again, starting from determination of t.sub.contr according to formula (1).

(9) The invention herein described was used for Urengoi-Center (outside diameter is 1420 mm, operating pressure is 7.4 MPa) trunk gas pipeline renovation. Pipeline section having length 60 km was previously hydraulically tested. Drying operation was performed by means of Atlas Copco XRX566CD air compressors (capacity is 2000 m.sup.3 per hour @ 0.1 MPa) and Atlas Copco CD 520 absorption units (dew point minus 40 C. @ atmospheric pressure). Above mentioned pipeline section was fitted with the valve station located 30 kilometers from drying unit. Intermediate air drying device (Dry Xtreme ND-032, 1962 m.sup.3 per hour capacity) was installed at valve station by-pass line. During the pipeline drying process the section from the upstream end of the pipeline up to the valve station and the section from the valve station up to the downstream end of pipeline) were dried simultaneously due to intermediate air dehydration up to the initial moisture content. As a result of the application of the proposed method providing an intermediate dehydration of the drying agent (air), the duration of the pipeline section drying process is 10.3 days, i.e. the said duration is reduced by approximately 1.8 times in comparison with the drying process in according to the commonly known method.