how to get desired line in the frame or measure the distance between two line?

edit

hello,

I an using this code for hough line

#include "opencv2/highgui/highgui.hpp"
#include "opencv2/imgproc/imgproc.hpp"
#include <iostream>

using namespace cv;
using namespace std;
Mat frame;
int main()
{
   Mat src = imread("building.jpg", 0); 

    Mat dst, cdst;

    Canny(src, dst, 50, 200, 3); 
    cvtColor(dst, cdst, CV_GRAY2BGR); 

    vector<Vec2f> lines;
    // detect lines
    HoughLines(dst, lines, 1, CV_PI/180, 150, 0, 0 );

    // draw lines
    for( size_t i = 0; i < lines.size(); i++ )
    {

        float rho = lines[i][0], theta = lines[i][1];

                if( theta>CV_PI/180*170 || theta<CV_PI/180*10)
        {
        Point pt1, pt2;

        double a = cos(theta), b = sin(theta);
        double x0 = a*rho, y0 = b*rho;

        pt1.x = cvRound(x0 + 1000*(-b));
        pt1.y = cvRound(y0 + 1000*(a));
        pt2.x = cvRound(x0 - 1000*(-b));
        pt2.y = cvRound(y0 - 1000*(a));


        line( src, pt1, pt2, Scalar(0,0,255), 3, CV_AA);
        }

    }

    imshow("source", src;
    imshow("detected lines", cdst);

    waitKey();
    return 0;
}

I want line aroung the tube only and other i don't want. the width of tube is aroung 46 pixel, from the frame. is there any suggestions how to detect the desired line.

Thank you very much in advance.