Need help in projected area of pipe along the direction of flow

Question

I have modeled pipe and elbow using MATLAB (Fig. 1). Have some confusion how to calculate the projected area of the element of pipe and elbow. For instance, if water is flowing in Z direction, elements of pipe1 and pipe 3 will not experience any pressure and force, while pipe 2, elbow 1 and elbow 2 will experience water pressure and corresponsidng forces. I was told that I have to calculate projected length and width and multiply to get projected area. This area will be such that when water is flowing in Z direction, corresponding area of pipe 1 and pipe 3 will be zero and there would be area for the remaining parts.

Fig. 1

My code is as follows:

%% element area pipe 1
for m = 1:size(element_num_shell_1_1,2) 
% size(element_num_shell_1_1,2) = 840, total number of elements of pipe 1
    if mod(m,n_ele_circum-1)~= 0 
% n_ele_circum = number of elements around the circumference = 15
        P1 = node_coord_list(m,[2:4]); % 
        Q1 = node_coord_list(m+1,[2:4]);
        R1 = node_coord_list(m+(n_ele_circum-1),[2:4]);
    else
        P1 = node_coord_list(m,[2:4]);
        Q1 = node_coord_list(m-(n_ele_circum-2),[2:4]);
        R1 = node_coord_list(m+(n_ele_circum-1),[2:4]);
    end
    if flow_direction == 1 % flow_direction =1 means flow paralle to + Z axis
        l = abs(P1(1)-Q1(1)); % length
        w = abs(P1(2)-R1(2)); % width
      elseif flow_direction == 2 % flow_direction =2 means flow paralle to + X axis
         l = abs(P1(3)-Q1(3)); % length
        w = abs(P1(2)-R1(2)); % width
    end
    Area_ele(m) = l*w; 
 end

Here, P1, Q1 and R1 are as follows in Fig. 2: Fig. 2

The problem is that, for pipe 1 and pipe 3, I got zero area, however, for pipe 2 and elbow also I got zero projected area. Can anyone help me in this regards? How actually we have to calculate prjected area of the elements of pipe and elbow in the direction of flow (say Z axis). Thanks.